Frits

2.11.2 Melting furnaces used in frits production

Continuous melting furnaces are mostly used in the ceramic frits industry, while discontinuous furnaces are rarely used. The choice of the type of furnace depends on the scale of production and the product formulation. The usual process involves producing a wide range of frit formulations in small melting furnaces, requiring high flexibility in order to adapt to the frequent changes in production. Most modern frit furnaces are generally operated with natural gas and there are different combustion possibilities depending on the oxygen content used for the combustion. In addition to traditional natural air-gas combustion, a significant number of furnaces use oxy-fuel combustion (mainly in Italy), accounting for about 15 % of the total furnaces in Europe. The enrichment of combustion air with variable amounts of oxygen, in order to provide a higher temperature in the melting furnace and, eventually, increase the production rate, is widely used particularly in Spain. The selection between different combustion options depends on the type of formulation/product and on the temperature needed for the melting process.

Furnaces for the production of frits are normally operated under a slightly negative pressure to ensure both an oxidising atmosphere and the flowing of the melt. This condition allows parasite air to enter the furnace, making it difficult to optimise the fuel/air (oxygen) ratio for environmental purposes.

Most furnaces are equipped with a heat-recovery system, except for the oxy-fuel fired ones. The combustion air is preheated up to 470 – 570 C. After the heat exchanger, the temperature of the flue-gases is still too high for entering a depollution unit (normally a bag filter), therefore an addition of fresh air is necessary for cooling.

In most cases, the flue-gases released by the melting furnaces are collected to a single depollution system or they are grouped, on the basis of the characteristics of the batch formulations, to a number of depollution units present at the installation. The combination of flue-gases from different furnaces using diverse combustion techniques (oxy-fuel, enrichment with oxygen, fuel/air) results in a flue-gas at the stack with a high concentration of oxygen, which is normally between 14 and 19 %, but can be higher.

Typical melting furnaces for frits production, with oxygen-enriched, fuel/air combustion, and heat recovery and oxy, and oxy-fuel combustion are shown in Figure 2.17 and Figure 2.18.

Melting furnace

Burner

Baghouse Ambient air

Ambient air

Ambient air T= 700 – 900 °C

T= 1300 – 1600 °C

Exchanger

Hot air

T= 170 – 200 °C

A

Oxygen Fuel

Safety valve

B

Figure 2.18 Schematic representation of typical melting furnaces for frits production with oxygen-enriched fuel/air combustion and heat recovery

Melting furnace

Burner

Baghouse Ambient air

Ambient air

T= 1300 – 1600 °C T= 170 – 200 °C

A

Oxygen Fuel

B

Figure 2.19 Schematic representation of typical melting furnaces for frits production with oxy-fuel combustion

Raw materials used for the preparation of the batch composition may be stored in silos and conveyed to the weighing area pneumatically or mechanically. Only in a few cases and due to the relatively small size of some manufacturers, some raw materials are stored in bags and manually dosed to the weighing apparatus. The various raw materials are automatically and precisely weighed and mixed to produce a batch that is chemically and physically uniform before being charged to the furnace.

In the continuous melting furnaces, the raw material is loaded by means of a worm screw forming a pile at the loading point. The burners, located along the sides of the furnace, provide the suitable thermal conditions for stability, allowing the pile of batch composition to melt continuously. Smaller melting furnaces need to be fuelled at one end through a simple burner.

As the material melts, a shallow layer is built up on the base of the furnace, which flows through the outlet at the other end of the melting furnace. Production remains constant due to the continuous feeding of raw material at the entrance.

The melted material can be fed directly into a water bath or cooled down between water cooling drawers in order to produce a fritted material.

Melting furnaces are shaped as boxes or as cylinders with lined up refractory bricks mounted on supports in such a way that enables a certain degree of rotation. In order to prevent contamination, the furnaces are usually dedicated to types of formulations with similar characteristics.

The typical melting temperatures are in the range of 1350 – 1550 °C, where the lower temperatures are used for ceramic frits with a low melting point and containing melting fluxes in their composition. The residence time in the furnace is determined by the time required to achieve complete conversion of the batch components into a melt.

2.11.3 Frits as raw material in the production of glazes and enamels

Glazes are manufactured by mixing the raw materials with one or more finely milled frits.

Milling is generally carried out in alumina ball mills with water. Other components of glazes, such as kaolin, colouring agents, electrolytes and opacifiers, need to be added in the various stages of the milling process. The time cycles at the mills range from between 6 and 16 hours.

After the milling operation, the mixed material is fed on a mesh screen and over a magnet in order to remove metallic impurities.

For dry products, the resulting material needs to be dried up or otherwise a dry-milling process may be used.

3 PRESENT CONSUMPTION AND EMISSION LEVELS