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BENEFITS OF UTILIZING NEPHELINE SYENITE IN PORCELAIN TILE AND SANITARYWARE Anais do 48º Congresso Brasileiro de Cerâmica Proceedings of the 48th Annual Meeting of the Brazilian Ceramic Society 28 de junho a 1º de julho de 2004 – Curitiba-PR
SCOTT E. BARTON UNIMIN CORPORATION 500 WILSON PIKE CIRCLE, SUITE 127 BRENTWOOD, TENNESSEE 37027 USA
EXECUTIVE SUMMARY
Nepheline syenite is a commercially prepared, rare feldspathoid-rich rock product that contributes both potassium and sodium oxides to the ceramic formulation. It has been used in both North America and Europe for the production of porcelain tile and sanitaryware for many years. It is the unique mineralogy and high levels of alkalis of nepheline syenite that justify its use in ceramics. These characteristics promote lower firing temperatures that can enhance productivity and lower production costs. Depending on location, the use of nepheline syenite can lower raw material cost by allowing more silica to be used in the formulation. Additionally, the process control found in most nepheline syenite operations offers a controlled level of Fe2O3 that can help to improve fired color. The goal of this study is to illustrate the practical benefits of using nepheline syenite for the vitrification in a ceramic body formulation.
Keywords: nepheline syenite, porcelain, tile, feldspar, sanitaryware
INTRODUCTION
As the porcelain tile and sanitaryware industries continue to globalize, manufacturers must continually improve the control and quality of their raw materials to effectively compete on the global scale. To do so, many manufacturers must look not only at domestic materials but also at external sources for these important requirements of quality and consistency. Nepheline syenite is one such product that is utilized on a global basis due to these requirements. Used both in ceramic bodies and glazes, nepheline syenite can both reduce vitrification temperature and allow for increased usage of more economic raw materials. More importantly, nepheline syenite can enhance productivity and quality in most ceramic applications. This paper will introduce nepheline syenite as a raw material and investigate its uses and benefits in porcelain tile formulation. Comments will also be made on its effect in other ceramic systems such as sanitaryware. 2
BACKGROUND ON NEPHELINEAnais do 48º SYENITECongresso Brasileiro de Cerâmica Proceedings of the 48th Annual Meeting of the Brazilian Ceramic Society GEOLOGY 28 de junho a 1º de julho de 2004 – Curitiba-PR
Nepheline syenite is an igneous rock, formed in a silica-deficient environment that is somewhat similar in appearance to granite. It is typically found as an irregularly shaped intrusion or in ring structures. The texture of the mineral is granitic and normally medium to coarse grained. Typically the most commercially viable deposits are found in intrusive structures with good liberation from contaminants such as biotite, hornblende and magnetite. Most noncommercial nepheline syenite deposits are gray or dark in color and have high iron levels that severely limit their potential in industrial applications. Those deposits associated with ring structures are especially difficult to beneficiate due to the extensive mixture of contaminant minerals.
A few special ore reserves exist in Nephton and Blue Mountain, in Ontario, Canada, and on the island of Stjernøy in Norway where the ore is unusually light or near white in color. A third such deposit is located about 40 kilometers from Rio de Janeiro in the municipality of Duque de Caxias. Commercially, the Nephton/Blue Mountain deposit is an open pit operation owned and operated by Unimin Canada Ltd. The primary market for the Canadian material is the U.S. and Canada with ever increasing offshore usage due to industrial globalization. The Stjernøy deposit, owned and operated by North Cape Minerals AS, is both an underground and open-pit operation located on the island of Stjernøy in the Norwegian Arctic1. Primary distribution for the Stjernøy materials is in Western Europe with the majority of product used in ceramic and glass applications.
MINERALOGY
Nepheline syenite is typically composed of three primary minerals: the albite form of feldspar, the microcline form of feldspar, and nepheline. The Norwegian and Brazilian deposits also contain what can be classified as perthitic feldspar, meaning a combination of sodium and potassium feldspar that began crystallization in solution but upon cooling became incompatible, forming sodium and potassium-rich lamellae. The percentages of each vary depending on the deposit location. The typical ceramic grade nepheline syenite will be composed of 20 to 30% nepheline.
CHEMISTRY
Nepheline syenite is anhydrous sodium potassium aluminosilicate. The typical chemical
formulas for the components of nepheline syenite are: albite (Na2O•Al2O3•6SiO2), microcline (K2O•Al2O3•6SiO2), and nepheline (3Na2O•1K2O•4Al2O3•8SiO2). While
“pure” nepheline has a theoretical chemistry of (Na2O•Al2O3•2SiO2), it is normally found 3
to have a 1:3 substitution of potassium into the matrix. Table 1 gives the calculated chemical composition of nepheline compared to that of microcline and albite, the two Anais do 48º Congresso Brasileiro de Cerâmica th most common formsProceedings of feldspar. of the 48 (Note Annual Meetingthat SiO of the2 Braziliancontained Ceramic in Society albite, microcline, and 28 de junho a 1º de julho de 2004 – Curitiba-PR nepheline is not “free-silica”, because the silicon and oxygen are combined with Al as well as Na/K.) It is the nepheline component of nepheline syenite that separates its
performance from feldspathic minerals. This component boosts the concentration of alumina and alkali oxide levels higher than that found in typical feldspar sources.
Table I. Calculated Chemistries of Nepheline Syenite Components
Oxides Nepheline Albite Microcline
(Na3, K) AlSiO4 NaAlSi3O8 KAlSi3O8 Alumina 34.9% 19.4% 18.3% Silica 41.1% 68.7% 64.8% Soda 15.9% 11.8% Potash 8.1% 16.9%
Physical Properties
Nepheline syenite is a moderately hard feldspathoid mineral assemblage that is deficient in silica. As an igneous rock, nepheline syenite was formed from molten magma material from deep within the earth’s crust. Unlike the igneous rock granite,
which contains crystalline silica (SiO2), nepheline syenite was formed in nature only when the molten material was deficient in that chemical combination. This deficiency in silica means that the cooling molten material did not form crystals of quartz or other forms of crystalline silica, but instead only formed the components of nepheline syenite. The Mohs hardness of the rock is about 6. It possesses a fairly low index of refraction, in the range of 1.50 to 1.53.
Table II. Typical properties of nepheline Syenite
Particle Shape angular to nodular Specific Gravity g/cc 2.56-2.61 Mohs Hardness 6.0 Refractive Index 1.51-1.53
Figure 1. SEM photomicrograph of nepheline syenite grains at 5,000x.
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MATERIALS AND METHODS
Anais do 48º Congresso Brasileiro de Cerâmica The first step of this Proceedingsproject was of the to 48 thselect Annual Meetingthe raw of the materials Brazilian Ceramic for comparisonSociety and prepare 28 de junho a 1º de julho de 2004 – Curitiba-PR the formulations. The goal of the project was to illustrate the benefits of utilizing nepheline syenite in porcelain tile formulations. Three (3) nepheline syenite products
were identified from the three (3) main sources: Canada, Norway, and Brazil. Two (2) feldspar products were identified from the Brazilian tile market: F1 and F2. All of the fluxes were obtained in granular form, approximately –30 / +200 mesh. Additional materials necessary for the testing were ball clay and ground silica. Respectively Unimin Premier BL AF and Unimin Cerasil 200 were utilized. These materials were obtained in 200-mesh form.
Each of the fluxes were ground to 2% +/- 0.5% retained on 200 mesh in 5 liter capacity laboratory jar mills. After each material was milled, chemical and physical analyses were made for use in preparing the theoretical formulations. Those analyses can be found below in Table III.
Table III. Results of chemical and physical analysis of fluxes after milling
Norway Canada Brazil F1 F2
Residue (%) (+) 200 mesh 1.97% 2.26% 2.11% 1.63% 1.55% (+) 325 mesh 19.03% 18.96% 20.79% 17.58% 18.24% (-) 325 mesh 79.00% 78.78% 77.11% 80.79% 80.20% Total Grinding Time 390 min 420 min 286 min 260 min 290 min
SSA 1.10 1.09 1.23 1.32 1.69 PSA 20m 59.0 57.4 53.9 57.5 56.6 (-100 mesh) 10m 40.4 40.0 33.5 38.5 35.0 5m 24.1 23.8 18.9 21.9 20.9 2m 8.9 8.0 6.5 7.9 8.9 1m 2.9 2.9 2.5 3.6 4.4 0.5 m <0.1 <0.1 0.2 1.2 1.2
Chemistry (by XRF) SiO2 56.45 61.09 62.62 69.32 77.83 Al2O3 24.50 23.74 22.36 17.20 12.09 Fe2O3 0.11 0.08 0.10 0.17 0.29 TiO2 0.06 0.01 0.02 0.02 0.08 MgO 0.03 0.03 0.01 0.02 1.14 CaO 1.06 0.24 0.64 0.19 0.96 K2O 8.96 4.90 6.22 7.40 4.52 Na2O 7.89 10.51 8.53 4.27 4.63 Total Alkali 16.85 15.41 14.75 11.67 9.15 LOI 0.95 <0.05 <0.05 1.42 <0.05
Using these analyses, formulations were prepared in two (2) phases. The 1st Phase used a set formula based on 45% ball clay, 10% silica, and 45% flux. This formula was based on porcelain tile formulations actually used in the Brazilian ceramic industry. The 2nd Phase controlled the theoretical chemistry of the formulation by varying the flux and 5
silica levels, while holding the ball clay concentration constant. The formulations can be seen in Table IV. Anais do 48º Congresso Brasileiro de Cerâmica Proceedings of the 48th Annual Meeting of the Brazilian Ceramic Society 28 de junho a 1º de julho de 2004 – Curitiba-PR Table IV. Phase 1 and 2 Formulations
Formulations Phase 1 Phase 2 Canada Norway Brazil F1 F2 Canada Norway Brazil F1/F2 Premiere BL AF 45 45 45 45 45 45 45 45 45 Cerasil 200 10 10 10 10 10 25 27 23 10 Canada 45 30 Norway 45 28 Brazil 45 32 F1 45 22.5 F2 45 22.5
Total 100 100 100 100 100 100 100 100 100
SiO2 65.13 63.04 65.82 68.83 72.66 70.91 70.38 70.63 70.74
Al2O3 22.40 22.75 21.78 19.46 17.16 18.87 18.61 18.90 18.31
Fe2O3 0.62 0.64 0.63 0.67 0.72 0.62 0.62 0.63 0.69
TiO2 0.55 0.57 0.55 0.55 0.58 0.55 0.56 0.55 0.56 MgO 0.15 0.15 0.14 0.14 0.65 0.15 0.15 0.14 0.40 CaO 0.15 0.52 0.33 0.13 0.48 0.12 0.35 0.25 0.30
K2O 2.43 4.26 3.02 3.56 2.26 1.70 2.74 2.22 2.91
Na2O 4.78 3.60 3.88 1.97 2.13 3.20 2.26 2.78 2.05 Total Alkali 7.21 7.85 6.91 5.52 4.39 4.90 4.99 5.00 4.96 LOI 3.79 4.49 3.84 4.70 3.37 3.89 4.34 3.91 4.03 Total 96.21 95.51 96.16 95.30 96.63 96.11 95.66 96.09 95.97
A 57 mm circular steel die was used to prepare each disk. 40 grams of dry powder formulation was pressed to 20 metric tons of pressure. The moisture level was controlled to approximately 6% for each disk. In Phase 1, nine (9) disks of each formulation were prepared for 3 firing temperatures. In Phase 2, ten (10) disks of each formulation were prepared for 5 firing temperatures. The number was limited to allow for sufficient room in the kiln for firing all disks together. After drying at 1050C, each disk was weighed and each diameter was measured for calculating loss on ignition (LOI) and fired shrinkage.
The disks were then fired at the appropriate temperatures. Phase 1 temperatures consisted of 11700C, 12000C, and 12300C. Phase 2 temperatures consisted of an initial firing of 12000C, 12300C, and 12500C, followed by additional firings at 12100C and 12200C. All firings utilized a similar firing curve with a heating rate of 150C per minute, maintaining maximum temperature for 30 minutes. Following firing, each disk was again weighed and diameter measured for completing the loss on ignition and fired shrinkage calculations. The disks were then analyzed for fired color and water absorption.
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RESULTS AND DISCUSSION
Anais do 48º Congresso Brasileiro de Cerâmica Both the Blue Mountain,Proceedings Canada of the and48th Annual Stjernøy, Meeting ofNorway the Brazilian deposits Ceramic Society are currently producing 28 de junho a 1º de julho de 2004 – Curitiba-PR ceramic grade nepheline syenite under the trade names Spectrum and Altaflux, respectively. The nepheline syenite sample from Brazil was obtained from the Duque
de Caxias deposit near Rio de Janeiro. The Brazilian feldspar samples were obtained from a Brazilian porcelain tile producer. Due to potential differences in firing due to particle size, great care was taken in the preparation of the nepheline syenite and feldspar powders. By controlling the ground residue, each powder was produced to be as similar as possible.
The Phase 1 formulations compare the relative performance of the different alkali sources in a standard formula. This illustrates the comparative firing performance of the nepheline syenite and feldspar samples at various temperatures. The results of Phase 1 can be found in Table V. From these results, it was determined that the level of alkali needed to be normalized in the formulations to better compare the fired properties. It was obvious that the initial firing of 11700C resulted in over-firing of the nepheline syenite formulations. Additional firings were necessary to determine the firing range of the feldspar formulations, which was found to be between 12000C and 12300C. Table V. Averaged results from Phase 1
11700C Shrinkage LOI Water Absorption GEB L a b Canada 8.56% 4.44% 0.04% 36.23 68.54 0.24 11.32 Norway 8.84% 4.67% 0.07% 33.51 66.32 0.10 11.35 Brazil 8.13% 4.62% 0.10% 34.63 67.55 0.59 11.70 F1 6.88% 4.51% 2.76% 40.83 73.19 0.80 12.62 F2 6.36% 4.76% 4.77% 44.84 76.47 0.32 12.88
12000C Shrinkage LOI Water Absorption GEB L a b F1 7.79% 4.54% 0.43% 40.80 72.07 0.23 11.25 F2 8.02% 4.79% 1.34% 42.49 73.13 -0.24 10.93
12300C Shrinkage LOI Water Absorption GEB L a b F1 6.91% 4.52% 0.19% 38.79 69.38 -0.06 0.01 F2 7.97% 4.78% 0.32% 41.82 71.57 -0.44 9.59
The Phase 2 formulations were calculated using theoretical chemical analyses. Modifying the silica and flux levels, the nepheline formulations were normalized to a target total alkali level (5.0%). That target alkali level was calculated from blending F1 and F2 at a ratio of 1:1 for use as 45% of the formulation, which was indicated to be the ratio currently used in the industry. The modified formulas were fired from 12000C to 12500C. The results are found below in Table VI. The results from Phase 2 (Table VI) illustrated good comparisons of fired quality between the different formulations. Maximum vitrification was achieved at approximately 12200C. After that temperature, the shrinkages decrease and water absorption properties notably increase, indicating that there was bloating (expansion) in 7
the disk. The fired shrinkage and loss on ignition (LOI) were similar, with the F1/F2 blend having slightly lower shrinkage and slightly higher LOI. Anais do 48º Congresso Brasileiro de Cerâmica Proceedings of the 48th Annual Meeting of the Brazilian Ceramic Society 28 de junho a 1º de julho de 2004 – Curitiba-PR Table VI. Averaged fired results from Phase 2
0 1200 C Shrinkage LOI Water Absorption GEB L a b Canada 7.17% 4.54% 0.85% 43.96 74.24 -0.09 11.01 Norway 7.22% 4.60% 1.86% 44.49 74.85 -0.09 11.26 Brazil 6.98% 4.63% 1.16% 42.95 73.61 0.10 11.16 F1/F2 7.21% 4.72% 1.85% 41.25 72.31 0.05 11.13
12100C Shrinkage LOI Water Absorption GEB L a b Canada 8.24% 4.62% 0.33% 40.57 71.05 -0.12 10.11 Norway 7.93% 4.70% 0.37% 40.27 71.01 -0.29 10.25 Brazil 7.91% 4.70% 0.27% 40.00 70.70 -0.02 10.23 F1/F2 7.71% 4.82% 0.31% 37.88 68.95 -0.16 10.11
12200C Shrinkage LOI Water Absorption GEB L a b Canada 7.97% 4.66% 0.23% 41.50 71.58 -0.30 9.79 Norway 7.73% 4.74% 0.27% 39.15 69.99 -0.44 10.01 Brazil 7.83% 4.77% 0.24% 37.48 68.75 -0.16 10.23 F1/F2 7.53% 4.86% 0.23% 38.02 68.80 -0.15 9.74
12300C Shrinkage LOI Water Absorption GEB L a b Canada 7.20% 4.53% 0.30% 44.02 73.24 -0.27 9.64 Norway 7.02% 4.57% 0.52% 42.93 72.57 -0.26 9.81 Brazil 6.52% 4.58% 0.36% 41.71 71.50 -0.19 9.67 F1/F2 6.02% 4.66% 0.40% 40.20 70.44 -0.25 9.76
12500C Shrinkage LOI Water Absorption GEB L a b Canada 5.45% 4.53% 0.29% 44.34 73.29 -0.31 9.33 Norway 6.17% 4.61% 0.43% 43.45 72.70 -0.29 9.41 Brazil 5.58% 4.65% 0.47% 43.17 72.39 -0.33 9.35 F1/F2 4.72% 4.68% 0.76% 40.79 70.67 -0.39 9.51
The fired brightness of the nepheline syenite formulations was higher than the feldspar blend at virtually all temperatures. This improved fired brightness can mainly be attributed to the chemistry of the product. In commercial nepheline syenite deposits, iron-bearing contaminants such as biotite mica can normally be removed using
separation techniques resulting in a product with a controlled iron (Fe2O3) level. Since nepheline syenite contributes a greater concentration of alkali oxides to the batch, greater amounts of silica will be required. This increase in silica utilization will put a greater emphasis on obtaining a silica source low in coloring contaminants, such as iron
(Fe2O3). The fired brightness results can be found in Figure 2.
Table VII. Calculated Coefficients of Thermal Expansion
Coefficient of Thermal Expansion (x 10-6) 5000C 7000C Canada 7.34 7.84 Norway 7.59 8.11 Brazil 7.19 7.68 Feldspar 6.68 7.14
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Due to the major difference in the silica level of the nepheline syenite and feldspar formulations, thermal expansion was measured for the disks fired at 12100C. This was Anais do 48º Congresso Brasileiro de Cerâmica the first temperatureProceedings that the of thefired 48th Annualabsorption Meeting ofwas the Brazilian lower Ceramic than Society 1.0%. The calculated 28 de junho a 1º de julho de 2004 – Curitiba-PR thermal expansion coefficients are in Table VII above, and the graph is in Figure 3 below. The increased level of quartz in the formulation did increase the coefficient of
thermal expansion for the nepheline syenite formulations. Some minor modifications to glaze formula may be required to prevent excessive compression in the body-to-glaze fit.
Figure 2. Fired Brightness (GEB)
Figure 3. Fired thermal expansion of Phase 2 fired disks
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COMMENTS ON THE USE OF NEPHELINE SYENITE IN SANITARYWARE
Anais do 48º Congresso Brasileiro de Cerâmica th 2 In 2001, authors G. KleinProceedings and of B. the Ersen48 Annual wrote Meeting of of thethe Brazilian benefits Ceramic of Society using nepheline syenite 28 de junho a 1º de julho de 2004 – Curitiba-PR versus feldspar in sanitaryware formulati ons. The authors conducted a very complete analysis of using three (3) formulations: feldspar only, feldspar/nepheline syenite, and
nepheline syenite only. The levels of kaolinite, quartz, and alkali were controlled in the study. This resulted in the need to use additional quartz, as less nepheline syenite was necessary to achieve vitrification. Three (3) distinct advantages were observed using 100% nepheline syenite as the fluxing agent.
1. Relative amount of flux was reduced by 50%, substituted using quartz. 2. Higher mechanical strength after sintering. 3. Lower deformation during sintering.
The advantages above are clearly evident when the feldspar (alkali) source is of low quality, and/or when a good quality silica source is readily available. As in our study, the thermal expansion was found to increase slightly with the use of nepheline syenite due to the increased quartz level in the formula. This increased quartz level could be attributed as the reason for the lower fired deformation observed. In situations where energy costs are high, a nepheline syenite source in sanitaryware could serve to lower firing temperatures and decrease gas consumption in the kilns.
CONCLUSIONS
Nepheline syenite is a powerful flux for use in virtually any ceramic formulation. The levels of alkalis found in existing nepheline syenite deposits are higher than feldspar and can help to lower the relative amount of flux used by up to 50%. This permits greater use of ground silica, which is usually more readily available, lower priced, and with lower iron-bearing contaminants than feldspathic rock. Alternatively, using higher levels of nepheline syenite, lower firing temperatures or faster firing cycles can be achieved, lowering energy consumption and increasing productivity. Commercially available with controlled, low-iron levels, this material can be utilized to produce the whitest fired colors. Combine these benefits with lower fired deformation and higher mechanical strength after sintering and you have a world-class product that is currently being shipped globally from deposits in Canada and Norway. Nepheline syenite could prove to be a tool that assists South American sanitaryware and tile producers to improve quality and export sales.
REFERENCES
[1] Potter, M.J. 2002, Feldspar and Nepheline Syenite, United States Geological Survey, viewed 12 August 10
2003<
BENEFITS OF UTILIZING NEPHELINE SYENTITE IN PORCELAIN TILE AND SANITARYWARE
As the porcelain tile and sanitaryware industries continue to globalize, manufacturers must continually improve the control and quality of their raw materials to effectively compete on the global scale. To do so, many manufacturers must look not only at domestic materials but also at external sources for these important requirements of quality and consistency. Nepheline Syenite is one such product that is utilized on a global basis due to these requirements. Used both in ceramic bodies and glazes, Nepheline Syenite can both reduce vitrification temperature and allow for increased usage of more economic raw materials. More importantly, Nepheline Syenite can enhance productivity and quality in most ceramic applications. This paper will investigate the uses and benefits of this valuable raw material in porcelain tile, with some additional commentary on its effect in other ceramic systems such as sanitaryware and glazes.
Keywords: nepheline syenite, porcelain, tile, feldspar, sanitaryware