1. GENERALLY 

Microwave (MW) heating is known as a fast and energy saving technology. It is based on electromagnetic induction of heat within the glass bulk. The method requires glass to be transformed into a sufficiently good conductor that then accepts power from an electromagnetic field.


   2. MICROWAVE HEATING OF GLASS

Partial dissipation of the electromagnetic field:

                                 P = 2 p f e e" E²

P = the power of dissipation
f = microwave frequency (2450 MHz)
e = permitivity
e" = the loss factor of the material
E = the magnitude of the local electric field

Glass materials have a very low loss factor and, therefore, melting and processing them by microwave is difficult.

    3. INTERACTION OF MATERIALS WITH MICROWAVES

From the point of view of MW absorption glass is a low loss material transparent for MW. It can not be heated by MW at room temperature.

Interaction of materials with MW

    4. HOW TO HEAT THE GLASS BY MICROWAVES?

The key condition is to "activate" glass for microwave absorption. The target is
achieving vibration of positive alkali ions in negative charged interstitial 
position to act as a rotating dipole. 

Glass Activation

    5. METHODS OF ACTIVATION

There are several methods how to activate glass for MW energy absorption described in literature:

• pre-heating of glass to 500 - 600 ^oC
  (hybrid system) 
  
  -the electric preheating causes higher energy consumption and such 
  hybrid systems are very costly.
  

• Addition of susceptor 
  (a material which absorbs microwaves at room temperature)
  
  - the susceptors change glass properties.
  

•  Application of MW absorbing coating or hydratation
  
  - it has no significance for use in industrial scale applications.
  

• Dr.Hájek's world wide patent pending method
  
  - the special method for activation of glass is based on generation of a high intensity MW field to a minimum space to start melting locally. The melting process is then transferred to the whole batch volume.
  
  

    6. COMPARISON OF TEMPERATURE PROFILES ON GLASS

With this method, melting has an inverse temperature profile, i.e. glass is heated and melted from inside toward to the pot wall. With an increase of temperature the absorptive ability of glass also grows and it accelerates the melting process.

Conventional furnace (temperature 800^oC) Microwave furnace(power 4 kW)

TEMPERATURE PROFILE OF GLASS IN CONVENTIONAL FURNACE (TEMPERATURE 800^oC)

 

TEMPERATURE PROFILE OF GLASS IN MICROWAVE FURNACE (POWER 4 kW)

MELTING OF GLASS CULLET (24% PbO) HEATING OF GLASS BATCH (1 kg)

Melting of glass cullet (24% PbO) weight 4 kg, power 4 kW 

Melting of glass batch (weight 1 kg, power 2 kW)

    7. REACTION MECHANISM AND KINETICS

The melting reactions show a different course as follows:

Conventional process

SiO₂ + Na₂CO₃ slow -»Na₂SiO₃

MW process

SiO₂ + Na₂CO₃ MW fast -»SiO₂ + Na₂O MW fast-»Na₂SiO₃


Notes:
SiO₂ ………………… MW transparent
Na₂CO₃…………… MW absorbent

    8. CHARACTERISTICS OF MIROWAVE HEATING OF GLASS

A. Any kind of glass (cullet) can be melted.
The glass properties are saved during the process.

B. Any kind of raw batch material can be used.
The quality of glass can be changed (e.g. mechanical resistance, morphology, microstructure).

C. "Microwave stirring" of melted glass has been observed.

D. Minimal evaporation of volatile components (e.g. cadmium, selenium).

    9. APPLICATION OF MICROWAVE HEATING OF GLASS

•  in glassworks for laboratory purposes like modification or manufacture of new kind of glass or execution of quick melting of a batch sample after changing of the batch formula
  

•  manufacture of artistic products and replicas
  

•  decoration by different coloured glasses
  
  Summarizing: in laboratories, studios, ateliers, glassworks, exhibitions etc.
  

    10. SCHEME OF MICROWAVE GLASS FURNACE

    11. ENERGY SAVINGS

Fast heating - due to the heating from the middle to the walls irrespective of the low heat conductivity of glass.

Selective heating - only the sample of glass is heated leaving the cooler area unaffected.

Closed system - the glass is closed in the furnace and losses of heat are almost eliminated by thermal insulation.

    12. ECOLOGY ASPECTS

Environmentally friendly working atmosphere.
No exhaust gases.
Low or no gaseous or particulate emissions.
No or low evaporation of volatile components.

    13. CHARACTERISTIC AND BENEFITS

•  Energy efficient

• Reduces emissions of pollutants 
  (no NOx emissions, other emissions low)

• Fast heating up, volumetric heating, reduced heating time

• Different reaction mechanism and kinetics, selective heating

• Any form of glass (cullet or batch or pelletised batch) can be melted

• When using cullet, a glass with the same properties is obtained

• MW irradiation of glass melt has a significant effect on homogenisation of glass

• self stirring effect in glass
  
  

 

    14. SUMMARY

• MW technology can be considered as a revolutionary small scale glass making process.

• Any kind of glass, cullet or batch can be melted by microwaves.

•  Glass of superior quality can be obtained.

• As an innovative non-traditional melting process it will improve energy efficiency and environmental performance.

• MW melting will be of value in specialized applications.

• A radical change of present industrial practice is worth considering.

•  MW irradiation of glass melt has a significant effect on homogenisation of glass via - self-stirring effect in glass.

• A break-through in the glass industry is a question of when not if.