MATERIALS

Nepheline Syenite

General Information:

Chemical formula: K2O•3Na2O•4Al2O3•9SiO2

Other names: Feldspathic rock

Sources:
Nepheline Syenite Spectrum A270 (Canada)
Spectrum N45 (Norway)

Common forms:
The common forms or varieties of nepheline syenite include cancrinite nepheline-syenite, eudialyte-bearing nepheline-syenite, foyaite, litchfieldite, malignite, pegmatitic nepheline syenite, pseudoleucite syenite, alkali-feldspar nepheline-syenite, and apgaitic nepheline-syenite. 12

Ceramic Substitutes: soda feldspar

A feldspathic mineral mined in Canada and Norway. Nepheline syenite is a valuable material in ceramics, acting as an efficient flux while providing beneficial properties like whiteness, low firing temperatures, and stable thermal expansion to clay bodies and glazes.

Although feldspar-like in its chemistry, mineralogically it is an igneous rock combination of nepheline, microcline, albite and minor minerals like mica, hornblende and magnetite. It is found in Canada, India, Norway and USSR. This material has a big advantage over feldspar: It does not contain quartz.

Nepheline syenite is widely used in clay bodies. It acts as a flux, lowering the maturing temperature required for vitrification. In vitreous china bodies, it forms a vitreous mass with free silica, increasing cast rate and density. Nepheline syenite contributes high alumina without associated free silica, stabilizing the expansion of the fired body. Typical usage levels range from 25-50% in cone 4-6 porcelain bodies.3 4 5

The unique properties of nepheline syenite make it valuable in glazes as well. It serves as an efficient flux, melting at lower temperatures than feldspars 6 8, which expands the working range of glazes. Its sodium content can help prevent crazing or cracking in glazes.35 When used in larger amounts, nepheline syenite can act as an opacifier, giving glazes a milky appearance. However, being slightly soluble, it can cause issues like gelling in glaze slurries over time if not properly deflocculated.3

Beyond its fluxing abilities, nepheline syenite imparts whiteness to clay bodies and glazes. Unlike some feldspars, it does not contain free silica or quartz, improving the fired product's expansion stability. Lower amounts are needed compared to feldspars due to its high alkali content relative to silica. These properties make nepheline syenite a valuable material for achieving desired characteristics in ceramic products.3 4

Unimin’s idled nepheline syenite pit. (Image: Peterborough Examiner)

How it’s processed:

Nepheline syenite is typically processed through the following main steps:

Crushing and Milling: The nepheline syenite ore is first crushed and then milled to reduce the particle size, typically to a range of 420 to 75 μm for glass applications and even finer sizes like 75, 53, and 38 μm for ceramic applications. 7

Magnetic Separation: After comminution, magnetic separation is employed to remove iron-bearing minerals like magnetite and hematite present in the ore.7 This is crucial as the iron content should not exceed 0.1% Fe2O3 for glass-grade nepheline syenite.

Flotation: Some processes also employ flotation techniques after or in combination with magnetic separation to further remove impurities and concentrate the nepheline and feldspar minerals.9 8

Leaching: Certain processes utilize leaching, typically with acids like oxalic acid, to remove residual iron and other impurities from the magnetic separation/flotation concentrates to meet strict iron specifications for glass and ceramics applications.7 9

Demagnetization: The final concentrates may undergo demagnetization to remove any residual magnetic particles.6

Size Classification:The processed nepheline syenite concentrate is classified into specific size fractions (e.g. -36+200 mesh) as required for different end-use industries like glass, ceramics, paints etc.6

The key aims are to remove iron-bearing impurities to very low levels (typically <0.1% Fe2O3), concentrate the nepheline and feldspar minerals, and produce a sized product with high alumina (typically >23% Al2O3) and alkali (>14% Na2O+K2O) content suitable for glass, ceramics and other applications.

Permaculture Ethics

Earth Care

There are several potential environmental concerns associated with mining and processing nepheline syenite:

Land disturbance and reforming: Mining of nepheline syenite, like any mining activity, can lead to significant land disturbance and reforming of the landscape. 9 This can disrupt natural habitats and ecosystems.

Solid Waste Generation: The mineral processing of nepheline syenite involves various steps like comminution, magnetic separation, and flotation, as described above. These processes can generate solid waste in the form of tailings, which may contain residual valuable minerals or potentially harmful substances.

Water consumption and pollution: Mining and quarrying activities associated with nepheline syenite have the potential to cause water pollution.8 This could arise from discharge of mine effluents, runoff from waste rock piles, or leaching of contaminants into groundwater. While not explicitly mentioned, mineral processing operations often involve water usage for various purposes, such as slurry preparation, flotation, and washing. This can lead to high water consumption.

Air pollution from dusting: The nepheline syenite tailings or mining waste can be a source of air pollution due to dusting, posing a threat to the environment.10 Fine particulate matter can get airborne and contribute to air quality issues.

Environmental risks from tailings: The nepheline-containing tailings or waste from mining and processing can pose real environmental threats if not properly managed. 10 These waste materials need to be contained or stabilized to prevent air and water pollution.

Rejected mining proposals due to eclogical risks: The search results cite a specific example where a proposed nepheline syenite mining project in Armenia was rejected due to potential ecological risks during the Soviet era, though it is being revisited now.8

While nepheline syenite itself may not be inherently hazardous, the mining, processing, and disposal of large volumes of the rock and its waste products can lead to various environmental impacts. Proper environmental assessments, mitigation measures, and waste management practices are crucial to minimize these concerns.

People Care

While the search results do not explicitly mention harm caused to humans from nepheline syenite exploitation, the use of hazardous chemicals, high-temperature processes, and potential occupational risks during mining and processing could pose risks if not properly managed and regulated. However, more specific information on the safety aspects and potential human impacts is not provided.

Other Uses for Nepheline Syenite:

The ceramics industry is one of the main consumers of Nepheline syenite though others include the glass, paint, coatings and construction industries, taking advantage of its properties as a flux, filler and whitening agent.

Glass Industry: Acts as alumina source, increases resistance, durability, lowers melting point, reduces fuel consumption, requires specific size range and low iron content.

Paint, Coatings, Adhesives, Plastics and Inks: Performance enhancer, provides brightness, tint retention, weatherability, color, sheen uniformity, chemical resistance, durability. 8

Construction Industry: Used in floor tiles, sanitary wares, construction materials for strength and toughness. 12 13

FOOTNOTES