Mechanics of Glass Processes

Industrial processes are numerous and depend on the application (glazing, containers, fibres, etc.).

All processes require a furnace with a high capacity, a long production line for upstream operations (batching, melting, fining, forming and annealing), while downstream operations (reheating, forming, molding, coating, tempering, laminating) can be performed on-site or off-site.

All these operations require a deep understanding and application of mechanics. At elevated temperatures, viscosity controls the homogeneity of the glass melt, the flatness of the glass while the thermomechanical properties determine the strength of the refractories used to fabricate the tools. At lower temperatures the tribological resistances of glass products will control surface quality and strength of the manufactured objects (flat glass, containers).

Four primary operations for glass manufacturing are: batching, melting, fining and forming.

Batching, melting and fining operations are common to all glass manufacturing processes with some variations according to the furnace type. The forming and post process depend on the end product.

Batching

Batching encompasses raw material selection based on chemistry, purity, uniformity and particle size. The batch comprises sand for silica, while modifiers are introduced as carbonates instead of oxides to reduce energy costs. The batch selection is adapted to the end product, for instance fibre production requires more selection and finer raw material particle size than container production. Also raw material humidity is controlled using IR analyser on modern production lines; also, impurity concentrations (Fe, Ni) are checked. Recycling of the glass has become an important parameter at this step and allows energy gains since a lower energy is required to melt the raw materials mixed with the recycled glass. Recycled containers are separated according to their colour to optimize the achieved product. Also, recycling may introduce ceramic contaminants that undergo reactions with the glass melt and are present as inclusions in the finished product. Metal and organic contaminants create instability during glass processing (through reduction/oxidation reactions) and degrade the quality of the glass. Delivery, mixing and sizing processes are highly abrasive, and equipments contain metals and ceramic-coated wear surfaces. Therefore, contamination risks exist from the tools and, actually, nickel sulphide particles are believed to form from nickel contained in such tools and from sulphur impurities introduced by combustion and are responsible for the delayed fracture of tempered glazing.

(Glass furnace interior)

Melting

Melting consists of complex chemical and physical phenomena. A large energy is required to fabricate soda-lime-silica glass.

Glass furnaces are used for melting the raw material particles and for transforming these into glass. The low melting constituents (alkali oxides) melt and dissolve the higher melting constituents such as quartz and alumina. Different furnaces are used for producing containers, fiberglass, flat glass and speciality glass. They can be divided into those heated electrically and those heated primarily by combustion.

Often electrical heating is used in combination with fuel firing (so-called electric boost) to improve heating uniformity and melt efficiency and to reduce gas consumption and emissions that is a matter of prime importance in the context of global climate changes.

Other polluting emissions (NO₂, SO₂) are generated and are reduced improving combustion (using oxygen instead of air) and fuel quality.

Electrical heating is used extensively and exclusively in smaller speciality and fiberglass melting units because of its lower initial cost and low emissions as compared to combustion furnaces even though energy costs remain high.

This drawback is compensated by flexibility in particular when small production volumes are required for speciality products. Glass conductivity plays an important role in this process.

Most furnaces are combustion heated which can be further divided according to the method used to recover exhaust waste heat and the way fuel is burnt (with air or oxygen). Oxygen fuel technology offers several advantages even though requiring pure oxygen. Regenerators can be avoided, eliminating furnace superstructures.

Heat recovery is of utmost importance since only 10% is used for the melt while 70% is lost through exhaust. Exhaust waste heat recovery is performed using regenerators that alternately store and recover heat, the shift being about every 20 minutes.

(Float glass line)