Glass is an amorphous,
non-crystalline solid material recognised for its low-cost
suitability for many everyday uses including transparent panes
and receptacles. While the adaptability of glass to many
different products and applications is recognisable, the use of
industrial minerals in its formation is by no means limited -
especially considering the variations between glass type and
glass manufacturing process.
The most familiar and commonly
produced type of glass is soda-lime glass. This glass is used
widely in two main types - flat glass, designed for end uses
such as window panes and in the automobile industry, and
container glass, used to make receptacles such as tablewear and
bottles.
Soda lime glass is produced using
minerals such as soda ash, quartz (silica) and lime.
Glass can also be defined in a much
wider sense to include every solid that possesses a
non-crystalline, amorphous structure that exhibits a glass
transition when heated towards the liquid state.
In this wider sense, glasses can be
made of different classes of materials including metallic
alloys, ionic melts, aqueous solutions, molecular liquids and
polymers. For many applications, polymer glasses, such as
acrylic glass, polycarbonate, polyethylene terephthalate, are a
lighter alternative to traditional silica glasses.
Glass manufacturing is complex and
encompasses an enormous range of compositions and product
types. Materials for glassmaking can be classified in three
groups: glass formers, fluxes and stabilisers.
Soda-lime glass
Soda lime glass is composed of approximately 75% silica
(SiO2), sodium oxide (Na2O) from soda
ash, lime (CaO) and several other minerals including stibnite
(the main ore of antimony) and feldspar. The mix of minerals
used to make soda-lime glass is termed ‘batch’.
The glass is produced by melting
the batch in a glass furnace at temperatures of up to
1,675¡C (3,047¡F). The temperature of the furnace
is limited by the quality of the furnace superstructure
material, that is whether it can withstand higher temperatures,
and by the glass composition.
“In the process of melting
[solid to liquid] a viscous liquid is formed and the mass
becomes clear and homogeneous at temperatures above 1,000
¡C. When removed from the reactor, the glass acquires a
shape allowing handling. By controlling the temperature of
cooling, [this] avoids devitrification or
crystallisation,” Felix Antonio Martinez Sendoya,
Colombian Silica’s CEO of silica and business, told
IM.
Relatively inexpensive minerals,
such as soda ash, sand and feldspar, are added to the batch as
fluxing agents to reduce the melting temperature of quartz and
help to control the viscosity of glass.
For minerals such as feldspar
(p74), the alkali content acts as flux, lowering the
glass-melting temperature and thus reducing production costs.
Coloured glass, such as that used to make green and brown
bottles, is produced from raw materials containing iron oxide
(see pp.42-45).
A typical batch for clear glass
containers may consist of 155kg soda ash, 172kg limestone,
145kg feldspar and 5kg sodium sulphate. Flat glass batches
require about 115kg of soda ash per 450kg of silica sand used
[Industrial Minerals and Rocks, 7th
Edition].
“Importantly, the
manufacturing process [to make glass] is practically the same
for all types; [what] changes from one specimen to another is
the material. All of them have a greater or lesser proportion
of silicon atoms,” Sendoya told IM.
While the materials used to make
flat and container glass come from the same mineral batch, the
physical process used to create the finished glass differs
slightly.
For flat glass, the float process
is administered, while for containers, glass blowing is
performed. Float glass has a higher magnesium oxide and sodium
oxide content compared with container glass, and lower silica,
calcium oxide and aluminium oxide content. This means that the
quality of container glass is higher, mainly due to the fact
that it is required to be safe for use as homeware receptacles
(for chemical durability against water).
Quartz
(silica)
Industrial sand and gravel, often called silica, silica sand or
quartz sand, includes sands and gravels with high silicon
dioxide (SiO2) content. Silica is the major
ingredient in virtually all types of glass including
containers, flat glass, lighting glass, tableware, TV tubes and
screens, decorative glass, fibreglass, optical glass and vacuum
flasks. The commercial uses of glass (containers, windows,
vehicle glazing) contain between 70 - 74% silica, the ultimate
source of which is silica sand.
Glass manufacturers are principally
concerned with the chemical composition of silica sands,
particularly iron, chromite and other refractory mineral
contents, according to the British Geological Survey (BGS).
Quality requirements depend on the
type of glass being manufactured - whether it will be coloured
or clear - and to some extent on the requirements of the
individual glass manufacturer. Silica sand for colourless glass
containers generally has an iron content of <0.035%
Fe2O3 (ferric iron oxide), for flat glass
in the range 0.040 to 0.1% Fe2O3 and for
coloured glass containers 0.25% to 0.3%
Fe2O3.
Global sand resources are large and
widespread and somewhat unquantifiable. However, in terms of
production, 140m tonnes silica sand was produced in 2012,
according to the US Geological Survey (USGS). Some 49m tonnes
of this sand were produced in the US, with around 17% of this
being used to make glass.
While resources and production are
on a large scale, several limitations surrounding silica sand
exist worldwide including geographical distribution - which can
often be a costly and uneconomic practice - environmental
restrictions - related to potential health risks associated
with the use of silica sand - and quality requirements for use,
as outlined above.
Soda ash
The glass industry is the largest consumer of soda ash,
particularly the glass-container sector. Soda ash has been used
in glass manufacturing for more than 5,500 years
[Industrial Minerals and Rocks, 7th
Edition] and continues to be used in the industry
today.
Soda ash is a source of sodium
oxide that is used as a fluxing agent in container, fibre, and
speciality glass manufacturing to reduce the temperature at
which the raw materials, such as silica sand, melt. Soda ash
also decomposes into sodium oxide and carbon dioxide, which
rises through the glass melt and helps mix the ingredients.
In terms of supply, China continued
to dominate last year, producing 23m tonnes in 2011, according
to latest USGS data. The US followed, producing 14.5m tonnes in
2011, with a total value of approximately $1.3bn.
“China continues to be the
largest and fastest-growing market for soda ash. Due to its
size and the persistent over-capacity in the Chinese industry,
it also has a strong impact on important markets for soda ash
trade - Asia, and to a lesser extent, Latin America,” FMC
Corp., the Wyoming, US-based producer, told
IM.
However, while the usual players
are continuing to produce within expectations, there are new
entrants to the market, including some from developing
countries.
“In developing countries, the
soda ash [market] is growing by 5%; in the US and Europe demand
is stable. Total growth is 2m tpa - 1m tpa in China [and] 1m in
the rest of the world,” Novacarb, the France-based soda
ash producer, told IM.
Turkey is a country that emerged as
a key soda ash producer in 2012, with more than 2m tonnes of
capacity logged in 2011.
“China continues to lead the
way for new capacity coming on-stream, and Turkey has emerged
as a new player with good access to European and Middle Eastern
markets,” Dennis Kostick at the USGS told
IM.
Recycling
The recycling of glass has brought the cost of domestic
container glass manufacturing down by significant amounts as
cullet, the mixture of used glass broken down to be recycled,
is used to replace part of the silica requirements in a glass
batch. Cullet also has a much lower melting temperature -
around 20-25% below a conventional batch - further reducing the
cost by lowering the amount of flux required.
“Glass recycling is an
increasingly important input into container glass production.
[É] more than 68% [2010 data] of glass bottles and jars
put on the market are collected for recycling and most of them
[some 80%] are recycled in a bottle-to-bottle, closed-loop
recycling system,” the European Container Glass Federation
(Feve) told IM.
“[This means] that, once
produced, glass can be used over and over again without
down-cycling and by reducing use of new minerals and raw
materials and preventing waste generation. Glass is
mono-material and does not require any additional barriers to
preserve food and drinks, making its recycling particularly
easy. This is a good illustration of how a circular economy for
packaging can work,” Feve added.
For every tonne of recycled glass,
1.2m tonnes of virgin raw materials are saved in the melting
process, according to the federation. This is extremely
important for the container glass industry because by using
recycled glass also reduces the amount of energy used and
CO2 emitted.
“It has a big contribution to
the sustainability of the industry. That is why some of the
mineral suppliers (for example Sibelco) are investing in
recycling facilities (for example Pate),” Feve told
IM.
While this is positive news for
container glass manufactures, it is not so good for soda ash
producers, which have seen decreased demand since the worldwide
push to recycle got underway in earnest.
Feldspar
Feldspar acts as a fluxing agent in glass making and is used to
bring the melting temperature of silica down. It is also a
stabiliser. Stabilisers impart to the glass a high degree of
resistance to physical and chemical attacks. Fluxes are oxides,
including potassium oxide (K2O) and sodium oxide
(Na2O) and stabilisers can be oxides such as alumina
(Al2O3) [Industrial Minerals and Rocks,
7th Edition].
During glass manufacture, feldspar
has two functions, acting as a flux by providing alkaline
oxides (K2O and Na2O) and as a stabiliser
by providing alumina and calcium oxide. A typical batch for
container glass contains around 8% feldspar, according to
Roskill Information Services.
Feldspar is the most abundant group
of minerals in the Earth’s crust, forming about 60% of
terrestrial rocks. Feldspar reserves are found in various
countries, including India, Czech Republic, Portugal and Poland
among others. Turkey and Italy are the top two producing
countries, accounting for about 50% of the world production
with almost 10m tpa in 2012, according to USGS estimates.
Identified and hypothetical
resources of feldspar are more than adequate to meet
anticipated world demand, the USGS added, which states that
quantitative data on resources of feldspar existing in
feldspathic sands, granites, and pegmatites generally have not
been compiled.
“Ample geologic evidence
indicates that resources are large, although not always
conveniently accessible to the principal centres of
consumption,” it said in its most recent feldspar
report.
Other materials that can be used
for glassmaking include fluorspar, lime, chromite, olivine,
staurolite and zircon sands, according to the USGS.
Glass markets: handle with care
Soda-lime glass, while the most abundantly manufactured form of
glass, has experienced testing times during the past few years
as the global economic crisis significantly slowed the amount
of new building as well as the amount of manufacturing
worldwide.
Glass was hit hard by a crash in
the global housing market during the period 2008-2012. This
collapse, in turn, caused the prices for some minerals used to
produce glass - such as soda ash - to dip to the point where
prices for the mineral were as low as they can feasibly go.
Other glass-using industries, such
as automobile manufacturing, also took a knock, which again
filtered down to the raw materials demand level.
Despite this, international
chemicals company Solvay, the world’s number one synthetic
soda ash producer, said that a drop in demand for flat glass in
the construction and automotive industries was buoyed by strong
performance in the container glass sector.
Feve agreed with this statement,
telling IM: “The container glass industry
produces some 20m tpa glass. In terms of tonnage, the biggest
markets for the container glass industry is the beverage
sector. Beer, spirits and wines account for the largest share.
Overall, glass packaging represents one-third of the European
beverage market in units.”
“While the beverage market is
important, the food sector is increasingly offering an
important growth opportunity for glass packaging. This is
certainly linked to new market trends orientated by consumers
demand for high-quality, natural, organic food. More than any
other material, glass can best communicate to consumers these
product characteristics. Glass is a packaging reference for
brands and for consumers,” the federation added.
While some glass markets dipped in
2012, this year has already begun to show some promise. In the
housing sector, for example, 23 markets have demonstrated
better year-on-year statistics in Q3 2012 compared with Q3
2011, according to a report by Global Property Guide.
Recovery of the US housing market has been particularly
evident, with growth in Q3 2012 being the highest since Q2
2006, according to the Federal Housing Finance Agency
(FHFA), the Guide reported.
“Certainly our sector saw a
dip during the crisis years, but we see that we are now almost
back to the pre-crisis levels in terms of tonnage. According to
latest available figures [H1 2012], glass packaging production
volumes in Europe grew by 1.9%. The growth is in response to
the increasing demand on the domestic and outside EU markets.
This builds on the positive trend recorded in 2011,” Feve
told IM.
“Despite the unstable economic
and financial crisis that negatively affects the whole European
manufacturing sector, these records shed a positive light on
the stability and future prosperity of the European container
glass sector. This is certainly due to the fact that consumers
continue to have a strong preference for glass - 74% of
consumers would recommend glass as a packaging material
[Insites 2010],” the federation added.
Despite these encouraging results,
the impact of the global economic crisis is still evident
across the glass industry.
Increased production costs,
unilateral CO2 costs, fluctuating and unfavourable
exchange rates, and high labour costs hamper the cost
competitiveness at global level of the container glass sector,
Feve said,
“Combined, these challenges
delay long-term investment decisions and, rather, become
incentives for delocalisation of production sites and R&D
investments outside EU to more industry friendly environments
with lower costs,” it added.
Conclusion
The fact that glass is faced with economic challenges is viewed
as a minor hurdle for many working in the industry, even at the
industrial minerals level, as glass remains the number-one
option industrial minerals producers of borates, silica sand
and soda ash. This is because it is used for many industries
including automobile, tableware, cosmetics and
construction.
This continued market demand will,
in turn, increase the call for industrial minerals such as soda
ash, silica, limestone and fluxes including feldspar.
Glass is also classed as a material
of the future as it is recyclable, re-usable and
refillable.
“This makes it certainly a reference for the circular
economy - where there is a big change from the linear economy
model to the circular model. This is based on recycling, which
closes the loop,” Feve said.
Glass fibre
Glass fibre is made up of bulk, chopped fibres or strands of
glass and durable plastic resin. It is used in reinforcing
plastics and composites as well as other specialised electrical
and thermal applications.
Minerals used in the creation of
glass fibre are similar to those employed in the creation of
other forms of glass. These include silica sand, limestone,
soda ash, borates, kaolin, lithium minerals, potash, feldspar,
fluorspar and sodium sulphate.
The most common glass fibre is a
calcium-alumina borosilicate with an alkali content of less
than 1%. It is commonly known as e-type glass, since it was
originally developed for use in electrical insulation
systems.
E-type glass was created in the
1950s to insulate electronics. Today, uses for glass fibre
include mats, thermal insulation, electrical insulation, sound
insulation, reinforcement of various materials, tent poles,
sound absorption, heat- and corrosion-resistant fabrics and
automobile bodies.
Glass fibre is used in consumer
goods such as hot tubs, bath tubs and boats. It is also used in
water storage tanks, for pipes in the oil and gas industry, as
well as in households and offices, in roofing and
cladding.
Glass fibre production
Glass fibres are produced by running molten glass from a
direct-melt furnace into a platinum alloy bushing containing a
large number of small holes, from each of which a glass
filament is drawn.
Filaments for commercial use are
normally between nine and 15 microns in diameter. The filaments
are layered with an emulsion before being gathered into
fibres.
The fibres are strong and they have
excellent electrical properties. They are also resistant to
most chemicals and moisture wear. They are non-combustible,
with a melting point around 1,500ºC.
Glass fibre industry
The fastest-growing industry sector for glass fibre is in
insulation, as populations become more concerned with energy
conservation. The European Union’s push to improve energy
consumption by 20% by 2020 has seen many governments offer free
or subsidised insulation and this is welcome news for glass
fibre minerals such as kaolin and borates.
Outside of the EU, Russia is aiming
to reduce energy consumption by 40% in 2020 and China has
invested heavily in rebuilding ‘energy poor’ housing
and has pledged $23bn for energy efficient projects.
The Obama administration in the US
has launched ambitious energy-saving initiatives, including a
$2,000 tax credit for residential homeowners who insulate
properly and make their existing properties more energy
efficient.
In Australia, the government
pledged in 2009 to install free ceiling insulation in around
2.7m homes under the Energy Efficient Homes Package. However,
the Home Insulation Programme was later shelved in favour of a
new scheme and then abandoned in April 2010. The scheme was
controversial, due to worker fatalities while installing the
insulation, and costly, due to compensation payment.
Source: this section is taken from
the article On a roll: glass fibre featured in
IM’s October issue.
Borosilicate glass
Glass accounts for three
major borate applications: insulation fibreglass, textile
fibreglass, and borosilicate glass. These applications are in
addition to a multitude of minor end uses.
Insulation fibreglass
Insulation fibreglass is the principal insulating material
used in the construction industry. Composed of very thin fibres
spun from molten glass, its purpose is to trap and hold air.
Borates are incorporated into the formulation to aid melting,
inhibit devitrification and improve the aqueous durability of
the finished product. Borates also ensure that the insulation
fibreglass has adequate recovery after prolonged compression in
order to save transportation.
Textile fibreglass
When compared to insulation fibreglass, textile glass fibres
are coarser with higher B2O3 content.
This type of glass was originally used for electrical purposes,
and low sodium levels were important. Now, however, major
applications include reinforcements for plastics, yet the low
sodium tolerance still applies.
Borosilicate glass
Borosilicate glass is a general classification referring to
glasses having the common characteristics of containing a
relatively high level of B2O3. Borates
impart many distinct properties to borosilicate glass: thermal
shock resistance, chemical resistance, aqueous durability, and
physical strength.
Uses include oven-to-table
cookware, laboratory ware, pharmaceutical containers, lighting
lenses, tubing, and vacuum flasks. Nuclear waste encapsulation
relies on the high tolerance and low leaching of borosilicate
glasses.
Small quantities of borates
are used in other vitreous products including container glasses
and art glasses. Optical glasses, microspheres, glass-ceramics,
“Vycor” glass, sealing frits and high-tech glass for
space exploration applications all rely on boron to help melt
or to fine-tune the final product properties.
Source: Rio Tinto
Minerals