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)

Float Process

This process was introduced by Pilkington Brothers Ltd in the 1950s.

Firstly, the raw materials (sand, soda, lime) are continuously introduced

into the furnace, melted at 1500˚ C, homogenized by convection and fined to eliminate bubbles. The furnace contains typically 2000 tons of glass and produces every day 500 tons of glass. The viscous liquid travels onto the float at a temperature of 1100˚ C under a nitrogen atmosphere in order to prevent corrosion of the tin bath. Under these conditions the equilibrium thickness of the glass sheet is about 6mm so that the sheet has to be expanded or contracted by top rolls (or rollers working from the top of glass) to produce thinner or thicker glass sheets respectively.

Several pairs of top rolls are used; they are made of steel and are water cooled. Their rotation axis is horizontal and shifted out of the float axis to draw the glass ribbon. The range of commercial thickness is between 2 and 19mm. Glass with lower thickness (<2mm) is difficult to produce and fusion draw is preferred. The glass sheet is extracted from the float at 600˚C. At such temperature the glass ribbon is viscous enough to be drawn upward out of the tin bath.

This process has been a revolution for the glass industry because it offers very good optical quality without requiring any further operation, while polishing before the invention of the float process. Therefore, most flat glass is produced through this process. This implies that to produce curved, tempered, laminated glazing it is necessary to heat the glass again close to its transition temperature.

An important characteristic of float glass is the intrinsic difference between the surface in contact with the bath (tin side) and the opposite side (atmospheric side).

Дата добавления: 2015-01-03