دراسة جدوى فنية/هندسية/بيئية/اقتصادية/جيولوجية لانشاء مصنع زجاج متكامل (النسخة الانجيليزية)

مجموعة تكنولاب البهاء جروب
مصر
عقيد دكتور
بهاء بدر الدين محكود
المكتب الاستشارى العلمى
An Overview of Glass and Glassmaking
Glass is one of the oldest of all materials known and used by mankind
Obsidian, a form of glass, was used by man thousands of y ears ago to form knives, arrow tips, jewellery etc.

Exhibits on display at the Corning Museum reveal that glass artefacts were being produced in the Mesopotamian region as early as2,500 BC.

By the year 1,500 BC glass vessels were in widespread use for cooking and drinking.

All the major civilisations including the Venetian, Phoenician and Roman empires produced ornate glassware.


The Venetian empire relocated its glass industry onto the island of Murano (circa1000 AD) partly for fire safety reasons but principally to protect its glassmaking secrets.


To this day the island remains a world-renowned centre for fine glassware.

Large-scale glass manufacture began with the industrial revolution.

Soda ash, an important raw material, became available at an affordable price with the invention of the Solvay process.

The Siemens brothers developed the modern day regenerative furnace in Germany around 1867.

The mass production of glass containers began at the beginning of the 20th century and the modern automated bottle and jar making (IS) machine made its appearance in 1925.


Glass light bulb production was automated with the development of the
ribbon machine in 1926.

The large-scale production of flat glass was originally achieved by pouring the glass onto tables and rolling the plates to the required thickness before grinding and polishing each side.

Continuous plate production using water-cooled rollers was introduced in1925 by the Fourcault process.


The flat glass process was revolutionised in1959 when Pilkingtons introduced the float process in which the molten glass is floated onto a bath of tin thus removing the need to grind and polish the glass plates.


Today’s furnaces owe much to these early designs and the basic regenerative furnace is still at the heart of large-scale glass manufacture.


The major advances since those early days have been essentially confined to improvements in the refractory materials resulting in
longer furnace lifetimes and in achieving much improved thermal efficiencies.

Today a typical container furnace would operate continuously for a period of 8 years producing 300 tonnes per day of molten glass at a thermal efficiency of around 4GJ/tonne.

The cost of such a furnace would be of the order of £5 million.


Common Types of Glass
Glasses can be produced in an almost infinite variety with specialist properties to match.

However the vast majority of common items are manufactured from a few relatively simple glasses.


Soda-lime glass

is by far the most common, finding use in the manufacture of flat glass, most containers and electric light bulbs, and
many other industrial and art objects.

More than 90 percent of all glass produced is soda-lime glass.
The basic composition of the glass comprises approximately 72 percent silica (from sand), 13 percent sodium oxide (from soda ash), 11 percent calcium oxide (from limestone), and about 4 percent minor ingredients.


All glass container manufacturers use the same basic soda-lime composition and generally only employ 3 basic colours.


This greatly simplifies the recycling process and allows the different manufacturers to recycle one another’s products without difficulty
other than the need to practice colour segregation.



Borosilicate glass

is heat-shock resistant and better known by such trade names as Pyrex. It contains about80 percent silica, 4 percent sodium oxide, 2 percent alumina, and 13 percent boric oxide.

Glasses with this composition show a high resistance to chemical corrosion and temperature changes and as such finds uses in such products as ovenware and beakers, test tubes, and other laboratory equipment Lead glass, commonly called crystal glass, is made by substituting lead oxide for calcium oxide and often for part of the silica used in soda-lime glass.

Lead glass

is easy to melt and has such beautiful optical properties that it is widely used for the finest tableware and art objects.

In addition, lead oxide increases the electrical insulation properties of glass.

Glasses with even higher lead oxide contents (typically 65%) may be used as radiation shielding because of the wellknown ability of lead to absorb gamma rays and other forms of harmful radiation.


Fibreglass

comprises fine but solid rods of glass, each of which may be less than one-twentieth the width of a human hair.


These tiny glass fibres can be loosely packed together in a wool-like mass that can serve as heat insulation in house construction.

Alternatively they can be used like wool or cotton fibres to make glass yarn, tape, cloth, and mats and as such have a huge number of uses including: electrical insulation, chemical filtration, and fire-fighter’s suits.


Fibreglass can also be combined with plastics to extend its usefulness to such items as aeroplane wings and bodies, automobile shell and boat hulls.



Whilst apparently having very different physical properties fibreglass is not that chemically different from “normal” glass and the manufacturers are able to accommodate some common glass (typically window glass) in with their raw
Cathode Ray Tubes

Cathode ray tubes, familiar in televisions and computer monitors, are essentially made from 4 different glass component parts.

Each part has a different function and the required properties can only be achieved by using glasses of different compositions.

A typical CRT tube comprises the screen, the funnel, the neck and a glass frit or solder used to join the component parts together.

The screen is the largest item and glass composition will include high levels of barium, strontium, and zirconium.

The screen is however lead free.

The other glasses are all of similarly complex compositions but these do contain varying amounts of lead.


Optical Glasses

Glasses can be designed to almost any specified combination of optical properties of which the most important are the refractive index (representing the deviation of a ray of light striking the glass at an oblique angle) and the dispersion (the dependence of the refractive index on wavelength resulting in colour separation).


Glasses with high dispersion relative to refractive index are called flint glasses while those with relatively low dispersions are called crown glasses.

Typically flint glasses are lead-alkalisilicate compositions whereas crown glasses are soda-lime glasses.

Sealing Glasses

another application for which a large variety of glass compositions are used is sealing to metals for electrical and electronic components.


Here the available glasses may be grouped according to their thermal expansions, which must be matched with the thermal expansions of the respective metals so that sealing is possible without excessive strain being induced by the expansion differences.


For example, sealing to tungsten in making incandescent and discharge
lamps, borosilicate alkaline earthsaluminous silicate glasses are suitable.

Sodium borosilicate glasses may be used for sealing to molybdenum or the iron-nickel-cobalt (Fernico) alloys that are frequently employed as a substitute, the amount of sodium oxide permissible depending on the degree of electrical resistance required.


Glasses designed to seal Kovar alloys require relatively high contents of boric oxide (approximately 20%) which keeps the transformation temperature low and in this case the preferred alkali ispotassium oxide which ensures high electrical insulation.



Where the requirement for electrical insulation is paramount, as in many
types of vacuum tube and for the encapsulation of diodes, a variety of
lead glasses (typically containing between 30% and 60% lead oxide) can
be used.


Special Glasses

Glasses with specific properties may be devised to meet almost any imaginable requirement, the main restrictions normally being the commercial considerations, i.e. whether the potential market is large enough to justify the development and manufacturing costs.

For many specialised applications in chemistry, pharmacy, the electrical and electronics industries, optics, the construction and lighting industries, glass, or the comparatively new family of materials known as glass ceramics, may be the only practical material for the engineer to use.

مجموعة تكنولاب البهاء جروب
مصر
المكتب الاستشارى العلمى
عقيد دكتور
بهاء بدر الدين محمود


The Glass Making Process

The basic (large-scale) manufacturing process and individual stages of glassmaking are illustrated below:


1-Prepare Batch

2-Melt & Condition

3- Form

4- Anneal

5-Inspect & Pack

6-Store & Dispatch


Batch Preparation

The composition of all commercially produced glass is very carefully controlled.

This is achieved by purchasing relatively pure raw materials and ensuring that they are well mixed in precise proportions before being fed to the melting furnace.

The major raw materials used in large-scale container and flat glass manufacture and their typical purchase costs are:

Raw Materials % £/tone
Sand 60 20
Sodium Carbonate 21 100
Limestone 19 30


In addition to these basic ingredients several other items may be added in order to bring colour or to impart improved chemical or physical properties.

Common additions include:

Additive Effect on Basic Glass
Iron Brown or green colour
Chromium Green colour
Cobalt Blue colour
Sodium Sulphate Improved refining
Lead Refractive index
Alumina Improved durability
Boron Improved thermal

The cost of minor ingredients can be substantial.

Selenium used to “whiten” clear glass costs in the order of £2,800 per tonne.

Fortunately only very small additions of these materials are required so their effect on the overall batch cost are low.

The typical batch costs of a commercial container glass are of the order of £40 per tonne of glass produced.

Soda ash (sodium carbonate) being the most expensive ingredient and also subject to the greatest price fluctuations.

Recycled glass (cullet) is also added to the melt.

The cullet may arise from within the factory as a result of breakage or rejected ware.

Cullet from this source is termed “domestic” and has the advantage of having an identical composition to the glass being melted.

Typically a container plant will reject or lose around 10% of its output and all of this will be recycled as domestic cullet.

Cullet brought into the factory from external sources (e.g. bottle banks) is termed “foreign.”

The exact composition of this cullet will be unknown and cannot be readily determined.

However, as most manufacturers use similar compositions, mixing this foreign cullet into the local glass composition of the same colour should present few problems.

Unfortunately this foreign cullet is often found to contain unwanted items such as metals, ceramics and “pyrex” type glass.

These items can either discolour the glass, pass unmelted through the furnace and cause a defect in the final glass product or, can even damage the lining of the furnace.

Dependent upon the level of contamination a furnace could operate at cullet (recycling) levels of over 90%.

An occasionally contentious point that can limit the level of cullet addition is the customer colour specification.

Many customers insist on colour specifications that the glassmakers feel is unnecessarily exacting with respect to the final application.

As the foreign cullet is the inevitable source of any trace contamination the effect of this policy of over-specification is a reduction in the recycling capacity of the glass plant.


Glass Melting


The raw materials and recycled glass are fed to the glass-melting furnace.

In large-scale operations the furnace is basically a refractory box-like structure which operates at temperatures up to1,600°C.

The furnace operates continuously providing glass 24 hours a day 7 days a week and all activities within the factory are entirely dependent upon its output.

A furnace is designed to operate a “campaign” lasting typically10 years before it is demolished and rebuilt.

The cost of a furnace is obviously related to its size but a typical300 tonne per day container furnace would cost in the order of £6 million.

It is estimated that there are currently 45 such furnaces operating in the UK varying in size from < 50 to > 700 tonne per day.

Small-scale operators may melt the glass in pot furnaces which typically hold less than 1 tonne of glass which is melted overnight ready for working the following morning.

The high temperatures required to melt the glass requires a lot of fuel and consequently the furnace is the main energy centre in any glass plant accounting for around 60% of the total plant demand.

Most furnaces are fired with natural gas but can also be fired on oil as a standby fuel.

An efficient furnace will require 4 GJ of energy for each tonne of glass melted.

Thus a furnace melting 300 tonnes per day will consume around 32,000 cubic metres of natural gas each day.


Once melted the batch material must be allowed time to thoroughly mix and allow any bubbles to rise.

The furnace thus has a large capacity and the batch material takes around 16 hours to pass through the melting stage.

The furnace is also the source of the majority of the airborne pollution that results from the factory.

The pollutants and the emission levels produced by a typical glass furnace are shown below.



Pollutant Typical Emissions


Dust (particulate) 80-140
Sulphur oxides 500-750
Chlorine (as HCl) 10-50
Fluorine (as HF) 1-15
Nitrogen oxides 1000-2000


Glass Forming

A great advantage of the glass manufacturing processes is that the finished article is produced immediately, as opposed to the production of an intermediate product that then requires transport to another factory for conversion e.g. steel sheets made for can manufacture.

though a hole at the end of these forehearths and is then directed into a series of iron moulds.

Compressed air is then used to blow the glass to the required shape.

The speed and scale of operation is impressive.

The forming machines serving a 300 tonne per day furnace must convert12.5 tonnes of glass each hour into bottles and jars.

Considering that the average bottle weight is some 284 grams the machines are producing in excess of 44,000 bottles per hour or over 7 million per week.

The melting furnaces producing flat glass for windows are much similar in design but much larger than those used in container manufacture and typically produce 700 tonnes per day of molten glass.

Once molten, the glass is formed into a single “ribbon” by floating it on a bath of molten tin.

The bath is connected to the melting furnace and produces sheets with a perfect surface finish.

700 tonnes of glass will produce70,000 m2 of standard window glass.

Fibre glass is produced by either drawing the glass through a bushing
containing dozens of tiny holes to form flexible fibres used for textile type applications or by using the centrifugal force of a spinner to form short fibres intended for insulation products.



Annealing

The forming process for rigid glass items involves some very rapid temperature changes and induces severe internal stresses within the glassware.

These stresses must be removed before the item is safe to handle.

The stresses are removed by the process of annealing, which involves reheating the glass followed by a controlled cooling cycle during which
the stresses are relieved.


The length of the annealing cycle is determined by the thickness of the item and can be of up to 40 minutes in duration.

The annealing process is performed continuously with the glassware on a conveyor belt being fed through a long tunnel kiln.

Inspection

All products leaving the factory are subject to some degree of inspection.

Originally a few items were sampled and tested in the laboratory.

The process is now highly automated and each item is subjected to a range of tests.

A simple beer bottle may have been through as many as 10 different checks before it is allowed to leave the factory.




Packing & Dispatch

With the very high production rate packing and dispatch is highly automated.

Many container customers operate a “just in time” policy so job planning throughout the factory is essential.


All plants have some warehousing but the container sector has such a diverse product range that it is impracticable to carry large stocks.