Frit

The molten material is then quenched causing the melt to solidify rapidly and shatter into friable particles termed frit. Glass frit is used as a raw material in the production of ceramic glaze. This is a vitreous coating applied to a ceramic body and fused by the application of heat. Similarly, enamel frit is a raw material used in the production of enamel. This is applied to metals for decorative and/or protective purposes. Glazes and enamels may be applied either dry or wet, the latter predominates and is usually in the form of a slip or slurry.
Important reason for using frits is melting rate. When raw glaze materials have been fritted, the reactions between them have already been largely completed. Therefore, there is less heat work required to fire the glaze coating. As a result, the surface of a high-frit glaze can be superior to that of a raw glaze of the same composition. Alternatively, the firing time can be reduced for a fritted glaze.
The process of fritting fuses water soluble raw materials into an insoluble glass, thereby making it easier to keep these materials uniformly distributed in the glaze or enamel suspension during subsequent processing. Furthermore, some of the raw materials used in the manufacture of glazes or enamels are both toxic and soluble. The conversion of these materials into an insoluble glass minimises the dissolution of toxic substances and therefore their potential for release to the environment.
Frits production does not present any significant specific emissions to air from downstream activities. Product grinding and milling is usually carried out wet, but dust control measures may be necessary if dry milling is carried out and potentially in dry product packaging areas. The most effective technique is likely to be extraction followed by a bag filter system.
Frit production utilises both continuous furnaces and batch furnaces. It is common for small batches to be produced for a wide range of formulations. Frit furnaces are generally natural gas or oil fired, and many frit plants use oxy-fuel firing. Continuous furnaces can be cross-fired or end fired with a single burner. Discontinuous batch furnaces are box shaped or cylindrical refractory lined vessels, mounted to allow a degree of rotation. The melt can be quenched directly in a water bath, or can be cooled between water cooled rollers to produce a flake product


Frit Manufacture:

Emissions of fluorides are directly related to the use of fluoride compounds in the batch. Fluorides are predominantly used in the production of enamel frits and are not usually present to any significant extent in the raw materials used for ceramic frit manufacture. Some ceramic frit producers may, periodically, manufacture small quantities of enamel frits in the ceramic frit kilns, giving rise to fluoride emissions, but this constitutes a very small proportion of the operator's overall production. The emission of fluorides is probably the most significant environmental impact of enamel frit production.
Fluorides provide unique properties to the frit, such as improved thermal and chemical resistance, and reduced risk of blistering of the enamel coating. They are added to the batch as fluorspar, fluorosilicate, cryolite or sodium fluorosilicate. Most producers now offer some fluoride free or low fluoride enamels, and the availability of these products is increasing. At the time of writing fluoride free products represent less than 10 % of production, with low fluoride products at about 30 %. The fluoride in the batch cannot generally, be reduced sufficiently to achieve emission levels comparable with scrubbing techniques or other frit types.

The use of cullet in a glass furnace can significantly reduce the energy consumption and its use is generally applicable to all types of furnace i.e. fossil fuel fired, oxy-fuel fired and electrically heated furnaces. Most sectors of the Glass Industry routinely recycle all internal cullet. The main exceptions are continuous filament glass fibre (where it is not considered possible due to quality constraints) and stone wool and frit production (where cullet as such is not produced). The base internal cullet level in the batch will usually be in the range of 10 to 25 %.

Oxides of Sulphur:
In general for frit production, the Best Available Techniques for oxides of sulphur (expressed as SO2) is considered to be fuel selection (where practicable) and control of batch composition. The emission level associated with Best Available Techniques is considered to be 0.1 - 0.5 kg/tonne of glass melted which generally equates to less than 200 mg/Nm3.
If oil firing is used or the batch contains significant levels of sulphate the emission level associated with Best Available Techniques is 500 - 1000 mg/Nm3.
Other Emissions from Melting.
In general in this sector, the emission levels associated with Best Available Techniques for the pollutants specified
below are considered to be:



Note1 ╺ Where batch materials contain significant levels of fluorides, this level is based on the use of an acid gas scrubbing system.

Integration of Frit Processes

A technique has been patented in Spain, which involves the designing of multi-furnace frit installations on a modular basis to group together similar types of operations. The approach allows the use of recuperators to preheat combustion air and also can make dust abatement and recycling more economical

Products and Markets

The principal application of glass frit is in the manufacture of ceramic glazes and pigments. These glazes, when applied to the surface of ceramic bodies such as tiles and tableware, and then fired, provide an impervious, protective and decorative coating. Frits may be sold in the pure form to the ceramic ware manufacturers who create their own glazes, or the frit manufacturers may produce and supply the glazes themselves. Across the sector typically over half the frit manufactured is used internally in the production of glazes.

Enamel frits are used in the manufacture of enamel glazes, the principal application of which is the coating of metal surfaces to provide a chemically and physically resistant covering. The principal market for enamels is in the manufacture of cooking equipment, and as a coating for hobs, ovens, grills etc. Other applications for enamels include storage tanks, silos, baths, electronic components and signs.
Frits are relatively high value, low volume products and transport costs generally comprise a relatively small proportion of the total product price. World-wide consolidation in the industry is resulting in relatively fewer but larger plants serving wider international markets.

Commercial Considerations

Production volumes have shown reasonable growth overall, with Spain showing an increase in sales in 1997 of over 15 %, a significant proportion of this is thought to have been in the EU. There is fierce competition between the producers in the EU both within, and between Member States. Most of the frits produced in the EU are used internally but exports are an important market for EU producers and are generally significantly higher than imports from non EU countries. The performance of the Frits Sector closely follows the success of the Ceramic Sector as a whole, and if the latter declines, then the Frits Sector that serves it, will also suffer. Imports into the EU of products using frits is potentially a greater threat than imports of frits themselves The production of glass frit is a well established industry and has served the ceramics sector for many years. Competition from raw glazes, which do not contain frit, is limited by their relatively poor leaching properties. Plastic coatings have been developed for tableware, but these suffer from the same leachability problems as raw glazes, particularly in the presence of organic acids, which are commonly found in food. It is not known to what extent plastic coatings may influence the market for fritted tile glazes. Threats to enamel glazes from substitutes are small. Alternatives, such as paints, could potentially be used in similar applications, but they cannot match the properties of enamels, in terms of heat, chemical and scratch resistance, and "cleanability".

Main Environmental Issues

The main environmental problem associated with frit production is that it is a high temperature, energy intensive process. This results in the emission of products of combustion and the high temperature oxidation of atmospheric nitrogen; i.e. sulphur dioxide, carbon dioxide, and oxides of nitrogen. Furnace emissions also contain dust (arising from the volatilisation and subsequent condensation of volatile batch materials) and traces of chlorides, fluorides and metals present in the raw materials. Frits produced for enamel glazes may result in substantial fluoride emissions due to the use of fluoride containing materials in the batch. Technical solutions are possible for minimising all of these emissions, but each technique has associated financial and environmental implications.
Water is used mainly for cooling and cleaning but also for shattering the molten glass to produce the frit (quenching) and for wet milling. Generally, these water circuits are closed circuits with a purge. Water used for quenching and milling may not need a purge, but will require the solids to be separated.
Waste levels are very low arising mainly from the solids collected from the water circuits. In many cases, waste from dust abatement equipment can be recycled to the furnace

Frits

Frit is prepared by fusing raw materials in a melter at high temperature. The molten material is then quenched causing the melt to solidify rapidly and shatter into friable particles termed frit. Glass frit is used as a raw material in the production of ceramic glaze. This is a vitreous coating applied to a ceramic body and fused by the application of heat. Similarly, enamel frit is a raw material used in the production of enamel. This is applied to metals for decorative and/or protective purposes. Glazes and enamels may be applied either dry or wet, the latter predominates and is usually in the form of a slip or slurry.
The process of fritting fuses water soluble raw materials into an insoluble glass, thereby making it easier to keep these materials uniformly distributed in the glaze or enamel suspension during subsequent processing. Furthermore, some of the raw materials used in the manufacture of glazes or enamels are both toxic and soluble. The conversion of these materials into an insoluble glass minimises the dissolution of toxic substances and therefore their potential for release to the environment.
The raw materials used in glass and enamel frit manufacture are essentially the same. They can be divided into four different groups, refractories, fluxes, opacifiers and colouring agents. Refractories include materials such as clay, feldspar and quartz. They are generally acidic in character and provide body to the frit. Fluxes are basic in character and react with the acidic refractories to form the glass. They include materials such as soda ash, potash, borax, cryolite and fluorspar.
Opacifiers provide the white opaque appearance that characterises many enamels. They can be insoluble such as titanium dioxide, tin oxide and zirconium oxide, or devitrification opacifiers such as cryolite or fluorspar. The latter may also act as fluxes rendering enamels more fusible. Opacifiers are not always included at the fritting stage, but may instead be added during slip production. Colouring agents may be oxides, elements or salts. Aside from their colouring properties they may act as either refractories or fluxes, and include materials such as cobalt oxide, chromium oxide and manganese oxide.
Raw materials may be stored in silos and conveyed to the weighing area pneumatically or mechanically. However, due to the relatively small size of some manufacturers many materials are stored in bags and manually dosed to the weighing apparatus. The various raw materials are precisely weighed and mixed to produce a batch that is chemically and physically uniform before being charged to the furnace.
The frit industry utilises both continuous furnaces and discontinuous batch furnaces. The choice of furnace is dependent on the scale of production and the product formulation. The nature of the business is such that it is common for small batches to be produced for a wide range of frit formulations. Frit furnaces are generally natural gas or oil fired, and most modern frit plants in Europe use oxy-fuel firing.
In continuous furnaces the raw materials are charged via a screw-feeder and form a pile at the charging point. Burners located along the sides provide temperature conditions of appropriate stability to enable the face of the pile to melt continuously. Smaller continuous furnaces may be end-fired with a single burner. As the materials melt, they form a shallow layer on the base of the furnace and flow to emerge at the opposite end. Production remains constant due to the continuous feeding of the raw material pile at the inlet. The molten frit can be quenched directly in a water bath, or can be cooled between water cooled rollers to produce a flake product.
Discontinuous batch furnaces are box shaped or cylindrical refractory lined vessels, mounted on bearings to allow a degree of rotation. To avoid contamination such furnaces are normally dedicated to similar types of formulation, though it is possible to purge furnaces between melts. Raw materials are charged through a port at the top of the furnace, and this can result in a short- term high level of particulate matter emission. Direct water quenching is used almost exclusively in batch manufacture, and the quench water may become contaminated with particulate matter and any soluble components from the melt.
Temperatures in the furnace are typically in the range 1000°C to 1500°C, though lower temperatures are used for high lead frits. During the melting operation metal fume and other particulates may be generated. Residence time in the furnace is typically less than 4 hours. To produce a slip, the frit must first be finely ground. Grinding is generally carried out in ball mills utilising alumina balls or flint pebbles in water. Further constituents of the glaze or enamel, such as clays, colours, electrolytes and opacifiers, may be added at any desired stage in the grinding cycle. Mill cycle times may vary from 6 to 16 hours. On completion of the milling operation the blended slip is passed over a mesh screen and over a magnet to remove tramp iron. For dry products the resulting slip may be dried or a dry grinding process may be used
Frits output

The main output is the product, and the melt yield from raw materials is 75 - 80 % depending on the precise formulation. Most of this loss is made up of CO2 emitted during melting. The processes do not produce cullet as such; and the only material generally recycled is dust from abatement equipment, which does not significantly affect the yield. The basic product yield from melt is very high because the material is simply quenched, the only losses are solids that cannot be separated from the water
Other Wastes

Waste levels are generally very low. The main processing waste is the solid material (mainly frit) separated from the water circuits. This material is not usually recycled because the composition is too variable. In most plants the waste to good production ratio will be in the region of 0.5 - 3 %.
Most mineral raw materials are delivered in bulk and do not give rise to packaging waste. Waste materials from product packaging operations (plastic, cardboard, wood etc) are usually reused or recycled if practicable. Other waste non-specific to the industry is disposed of by conventional means, or recycled where local or national schemes permit. At the end of a furnace campaign, the refractory structure is dismantled and replaced. Where practicable this material is recovered for reuse or sale.
Raw materials
In most modern container glass processes silos and mixing vessels are fitted with filter systems which reduce dust emissions to below 5 mg/m3. Mass emissions from both filtered and unfiltered systems will depend on the number of transfers, granule size, and the amount of material handled. Although quite rare and only in low levels some frit processes involve the use of raw materials containing lead or other heavy metals. Where materials containing potentially toxic compounds are handled there is the potential for emission of these substances. Almost invariably effective measures will already be in place to minimise emissions of these substances.
The main raw materials used for the most common formulations are given in the below, along with an indicative batch composition.
Frit Type Raw Materials Approximate % in batch Ceramic/Glass Frits:
  1. Zirconium compounds → 7.7
  2. Feldspar → 26.8
  3. Quartz → 25.9
  4. Boric acid → 6.8
  5. Zinc oxide → 8.4
  6. Dolomite → 8.4
  7. Calcium carbonate →13.4
  8. Potassium nitrate → 2.6
  9. Enamel Frits Borax → 19.1
  10. Quartz → 42
  11. Sodium nitrate → 7.8
  12. Sodium fluorsilicate → 1.2
  13. Potassium fluorsilicate → 7.8
  14. Sodium phosphate → 3.2
  15. Titanium oxide → 18.9
  16. Low melting point frits Red lead Pb3O4 → 50
  17. Quartz → 19.8
  18. Zinc oxide → 15.1
  19. Boric acid → 15.1

Main raw materials utilised in frit production


Water is used for cooling and cleaning purposes, but also for cooling and shattering the molten glass (quenching) and for wet milling. All of the water circuits are usually closed circuits with corresponding losses from evaporation and purges. Other water losses are the water content of the product and the water content of the solids collected from the quench water circuit. Water consumption is estimated at 0.5 - 1.5 m3/tonne of frit.
The main fuel used is natural gas, oil is also used in some installations but much less commonly (<10 %). Many installations are oxy-gas fired and so consume a relatively large amount of oxygen.