Silica sand and glass industry

Topic 6: GLASS

A short series of lectures prepared for the Third year of

Geology, Tanta University

2013- 2014

by

Hassan Z. Harraz

[email protected]

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OUTLINE OF TOPIC 6: Glasses

Raw Materials:

a) Silica sand b) Limestone c) Soda ash

Glass Manufacturing Process

Glass Forming

Glass Structure

Glass Properties

Glass Types:

i) Soda-lime glasses

ii) Lead glasses

iii) Heat-resistant or borosilicate glasses

iv) High-purity silica glasses v) Specialty glasses

Heat Treating Glasses:

a) Annealing glass b) Tmpered glass

Chemistry of Glass Manufacture

Recycling of Glass

Virtification

Question

What is Glass? Glass is an amorphous solid. A material is amorphous when it has no long-range

order, that is, when there is no regularity in the arrangement of its molecular constituents on a scale larger than a few times the size of these groups. [...]. A solid is a rigid material; it does not flow when it is subjected to moderate forces - Doremus

Glass includes all materials which are structurally similar to a liquid. However, under ambient temperature they react to the impact of force with elastic deformation and therefore have to be considered as solids. -Pfaender

Glasses have numerous properties in common with crystalline solids, such as hardness and elasticity of shape [...]. The term 'amorphous solid state' has a more comprehensive meaning broader than that of the 'vitreous state'. All glasses are amorphous, but not all amorphous substances are glasses. Feltz, 1993

13 March 2014 Prof. Dr. H.Z. Harraz Presentation Glass 3

Glasses Clay products

Refractories Abrasives Cements Advanced ceramics

optical composite

reinforce containers/

household

-whiteware - bricks

-bricks for high T

(furnaces)

-sandpaper - cutting - polishing

-composites - structural

engine - rotors - valves - bearings

-sensors

Adapted from Fig. 13.1 and discussion in Section 13.2-6, Callister 7e.

Taxonomy of Ceramics

13 March 2014 13 March 2014 Prof. Dr. H.Z. Harraz Presentation Glass 4

Glasses A glass can be defined as an inorganic product which has

cooled to rigid structure without crystallization. Glass is hard material normally fragile and transparent

common in our life. Glass-ceramics have an amorphous phase and one or more

crystalline phases and are produced by a so called "controlled crystallization" in contrast to a spontaneous crystallization

It is composed of mainly: Sand Alkali

Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass manufacturing process. The glass is cooled down and is then reheated in a second

step. In this heat treatment the glass partly crystallizes Two prime characteristics of glass are their optical transparency

and the relative ease with which they may be fabricated. Amorphous solid materials No crystal structure No long-range order Resemble “frozen liquids”


RAW MATERIALS

Raw Materials Approximate

Proportion (wt %)

Provides Approximate Proportion

in glass (wt %)

Soda ash (Na2CO3) 25 Soda (Na2O) 18

Limestone (CaCO3) 10 Lime (CaO) 7

Silica sand (SiO2) 65 Silica (SiO2) 75

Raw materials used in lime-soda glass

a) Silica sand

Silica sand suitable for glass manufacture is however relatively rare, because of the need for a high degree of chemical purity.

The essential requirements for silica sand for glass manufacture are that it must be even grain size - more than 90% of grains must lie in the range 125-500µm, and its chemical composition must meet the requirements shown in Table 4.

Maximum

Cr2O3

Maximum

Fe2O3

Minimum

SiO2 Glass

0.00015 0.013 99.7 Opthalmic glass

0.0002 0.010 99.6 Tableware, crystal and borosilicate glass

0.0005 0.030 98.8 Colourless containers

-- 0.25 97.0 Coloured containers

0.0001 0.10 99.0 Clear flat glass

Table 4: Required chemical composition of silica sand for glass manufacture

Fig.1: High pure silica sand raw materials

a) Silica sand The discolouring impurities iron and chromium occur within the non-quartz mineral fraction of the

sands. Iron can occur as haematite, giving the sand a red colour, or as oxy-hydroxidcs (giving a yellow or brown

colour) as well as in silicate minerals. Chromium occurs as the heavy mineral chromite (FeCr2O4), which is stable during glass manufacture,

and so rather than resulting in a discoloured glass, it persists as solid inclusions within the finished product, which can cause it to be brittle. This is especially important for float glass manufacture, where persistence of chromite grains can render useless substantial lengths of glass strip. Because of the difficulties involved in the chemical determination of minor amounts of Cr it may be appropriate simply to count the number of grains of chromite detected optically within a sample of known weight in order to classify a sand as suitable for float-glass.

Alumina is a natural impurity in glass sands, arising from the presence of feldspars, mica or clay minerals, and varies from 0.4% to 1.2% Al2O3 High values in this compositional range are preferred because they help to reduce melting temperatures (yet another component is added) and involve no negative effect on glass colour or other physical properties. The occurrence of aluminium as an impurity may also be beneficial by reducing the need to add aluminosilicates (feldspar, aplite or nepheline syenite) for the manufacture of certain glasses.

Great care is taken to consider the minor components of a glass, as small traces of impurities may have a major positive or negative effect on the quality of the finished product. For example, the presence of traces of iron may give a pale green colour (often visible when examining a pane of glass end on), and this can be tolerated in some applications (such as container glass).

Other minor components might have beneficial effects on the qualities of the glass produced. For example, addition of lithium (reduces the temperature required to melt the glass, and so yields savings in energy costs.

b) Limestone • Limestone is required twice in glass manufacture - once to produce

sodium carbonate and secondly as an ingredient in the batch to be melted.

• As an ingredient in batches to be melted to produce glass, limestone purity is critical. In particular, Fe contents have to be very low, and the amount of MgO, as in dolomite, has to be known. In some glasses MgO is added using pure dolomite, but the amounts have to be controlled.

• Like CaO, MgO causes immiscibility in glass melts; the miscibility gap in the system SiO2-MgO is wider than that in the system SiO2-CaO (Fig.4).


Limestone Cycle

Limestone CaCO3

Lime

Hydrated

Lime

Milk of Lime

Heat

CO2

H2O H2O

CO2 H2O

Slurry

Calcium

oxide

(CaO)

Calcium hydroxide

(Ca(OH)2 - DRY

Calcium

Hydroxide

Ca(OH)2 - WET

Lime (CaO) Include hydrated lime & quicklime Only quicklime can use to make glass

Extraction of Lime Quarry of limestone Transported to crush plants Undergo Calcination process:

heating limestone or chalk (CaCO3) in kiln till 900oC CO2 is emitted in this process and calcium oxide

(lime) is produced.

Calcination Process


• Calcined lime Quicklime/Burnt lime/White wash is obtained by heating

limestone at temperatures above 900oC in a Kiln: CaCO3 heat CaO + 2CO2

• Hydrated lime Calcium Hydroxide/Slaked Lime is a dry powder, resulting from

the controlled slaking of Calcined Lime with water in a Hydrator: CaO + H2O Ca(OH)2

• Precipitated Calcium Carbonate Carbonation of Hydrated lime, also known as purified, refined or

synthetic Calcium carbonate: CaO + H2O + CO2 CaCO3 + H2O

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Vertical Lime Kiln


c) Soda Ash (NaHCO3)

Anhydrous sodium carbonate Texture: soft Color: grayish & white Appearance: lump / powder in nature

Naturally: Erosion of igneous rock form sodium deposits Transport by waters as runoffs & collect in

basins When sodium comes in contact water/ CO2,

precipitates out sodium carbonate.

Synthetically: Extraction of Soda Ash(NaHCO3), Manufactured synthetically through Solvary

process by using salt, ammonia & limestone


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The Solvay process for the manufacture of Soda Ash (NaHCO3)

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Impurity The Na2O and CaO decrease the softening point of this glass from 1600oC to 730oC So that soda lime glass is easier to form. An addition of 1 – 4% MgO is added to Soda lime glass to prevent cracks.

Magnesium can be substituted for a proportion of the calcium content by the use of dolomite instead of limestone

In addition of 0.5 – 1.5% Al2O3 is used to Increase the durability. Alumina is a widespread component of glasses in addition to soda ash and silica, and helps improve resistance to weathering.

Boric oxide (to produce heat-resistant glasses such as 'Pyrex' and 'Vycor') and Lead oxide (for lead crystal tableware). Potassium can be substituted for some of the sodium with the use of feldspar, aplite

or nepheline syenite. fluorides.: used to produce Opaque glasses . Lithium (Li2O) is added to the glass composition: The amounts required are very

small, frequently ~1 to <4%. Lithium is added to glasses for several reasons, because it reduces liquidus temperatures; it improves moulding properties (reduces viscosity); it improves thermal properties ('Pyrex', ceramic hobs) and it improves strength.


Ingredients To Obtain Glass

There are following main ingredients used in the manufacturing of glass:

Sand (SiO2), Quartz, or Silica sand 72%

Flux → to lower T – e.g. Soda or Soda Ash (NaHCO3) 17%; (1700 – 900oC)

Stabilizing agent → to mitigate water solubility of the glass formed – e.g. CaO normally added as Limestone {Lime 5%}

What is the raw material? Percentage of Ingredients in Glass

silica sand

soda ash

lime

other ingredients

72%

17%

5%

6%


Glass Manufacturing Process 1. Silica sand, limestone, soda ash and cullet (recycled glass or

broken glass) are keep dry and cool in a batcher house in silos or compartments

2. Mixing and weighting into proper proportion

3. Send to furnaces in hoppers:

operated by natural gas

heat the mixture at 1300-1600oC into soften or molten state

4. Molding (or Casting ): molten glass flows to forming machine to mold into desire shapes

5. Annealing lehrs : reheating the glass in an oven

to ensure even cooling of glass for strengthening of the products

6. Cooling process: Cool for 30 min to an hour for safe to handle.

7. Glass products are then decorated, inspected again and finally packaged and shipped to our customers.

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Glass Furnace Cooling Systems


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The Process


Glass Forming

Flat glass – floating / rolling

Glass fibre – continuous strands and Crown process for glass wool

1) Casting : molding 2) Pressing: pressing second mold into molten glass 3) Core-forming: clay core dipped into molten mass 4) Fusing : fusing glass rods together around a mold 5) Blowing: blowing air into a glob

13 March 2014

Prof. Dr. H.Z. Harraz Presentation Glass 18

Glass Fabrication Methods

• Pressing:

GLASS FORMING

Adapted from Fig. 13.8, Callister, 7e. (Fig. 13.8 is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.)

Gob

Parison mold

Pressing operation

• Blowing:

suspended Parison

Finishing mold

Compressed air

plates, dishes, cheap glasses -mold is steel with graphite lining

• Fiber drawing:

wind up

PARTICULATE FORMING

CEMENTATION

Blow Molding

Softened

glass

Softened

glass

Pressed Glass Processing


Float Glass: The Process

Image from Prof. JS Colton, Ga. Institute of Technology

Modern Plate/Sheet Glass making:

Glass Structure

• Quartz is crystalline SiO2:

• Basic Unit: • Glass is amorphous • Amorphous structure

occurs by adding impurities

(Na+,Mg2+,Ca2+, Al3+)

• Impurities: interfere with formation of crystalline structure.

(soda glass)

Adapted from Fig. 12.11, Callister, 7e.

SiO 4 tetrahedron 4-

Si 4+

O 2 -

Si 4+

Na +

O 2 -


Glass Properties

Specific volume (1/r) vs Temperature (T):

• Glasses: do not crystallize change in slope in spec. vol. curve at

glass transition temperature, Tg

-- transparent - no crystals to scatter light

Crystalline materials: crystallize at melting temp, Tm have abrupt change in spec. vol. at Tm

Adapted from Fig. 13.6, Callister, 7e.

T

Specific volume

Supercooled Liquid

solid

T m

Liquid (disordered)

Crystalline (i.e., ordered)

T g

Glass (amorphous solid)


Glass Viscosity vs. T and Impurities

• Viscosity decreases with T • Impurities lower Tdeform

Adapted from Fig. 13.7, Callister, 7e. (Fig. 13.7 is from E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.)

Vis

cosi

ty [

Pa

s]

1

10 2

10 6

10 10

10 14

200 600 1000 1400 1800 T(°C)

T deform : soft enough

to deform or “work”

annealing range

Tmelt

strain point

• fused silica: > 99.5 wt% SiO2

• soda-lime glass: 70% SiO2 balance Na2O (soda) & CaO (lime)

• Vycor: 96% SiO2, 4% B2O3

• borosilicate (Pyrex): 13% B2O3, 3.5% Na2O, 2.5% Al2O3


Glass Types

Five common types of glass:

i) Soda-lime glasses

ii) Lead glasses

iii) Heat-resistant glasses OR Borosilicate

iv) High-purity Silica glasses

v) Speciality glasses

There are the following types:-

Fused silica glass

96% silica glass

Soda lime glass

Lead silicate glass

High lead glass

Boron silicate glass

Alumina borosilicate glass

Low alkali glass

Alumina silica glass

Glass ceramics


i) Soda-Lime-Silica Glasses • 65% sand; 15% soda; 10% lime • In this glass component are:

71 – 73% SiO2

12 – 14% Na2O 10 – 12% CaO

• Adding sodium oxide (soda) lowers melting point • Adding calcium oxide (lime) makes it insoluble • Sodium and calcium ions terminate the network

and soften the glass • The Na2O & CaO decrease the softening point of this

glass from 1600oC to 730oC, So that soda lime glass is easier to form.

• An addition of 1 – 4% MgO is added to Soda lime glass to prevent cracks.

• In addition of 0.5 – 1.5% Al2O3 is used to Increase the durability

• Soda-lime-silica glass is most commonly produced glass which accounts for ~95% of all the glass produced in the world.

• Soda-lime-silica glass expands much when heated Breaks easily during heating or cooling

Uses Soda lime glass is used for flat glass, containers,

lightening products. It is used where chemical durability and heat

resistant are not needed


ii) Lead Glasses

• Lime and soda replaced with lead oxide (PbO)

• Contains lead oxide (PbO) to improve refractive index

• High refractive index- clarity sparkle

• Softer –cut and engrave

• Good electrical resistance - electronics


iii) Heat-resistant (or Borosilicate) Glasses • Contains Boron oxide, known as

Pyrex. • Boron-oxide-silica glass expands

less Tolerates heating or cooling

reasonably well • Pyrex and Kimax are borosilicate

glasses • Boron oxide replaces lime and

most of soda – low thermal expansion coefficient

• Al2O3 - B2O3 – aluminosilicate glass with even better heat resistance


iv) High-purity Silica Glasses

• Highest quality – most durable • 3 processes – melting pure SiO2; making 96% silica and flame

hydrolysis • Pure SiO2 – pure silica melted @ 1900 ºC under vacuum

• 96% - Vycor process – borosilicate glass heated to grow crystalline sodium borate channels – extracted hot HNO3 – leaving 96% pure silica after heat reduction @ 1200 ºC

• flame hydrolysis – SiCl4 in CH4 / O flame (1500ºC, produces high-surface silica soot thermally sintered to pure silica at 1723 ºC)


2H2O + SiCl4 SiO2 + 4HCl Flame

v) Specialty Glasses • Coloured glass:

MnO2 – violet, CoO – blue, Cr2O3 - green

• Opal glass: white opaque or translucent glassware colour due to scattering of light from small particle usually NaF/CaF crystals nucleating after a cooling and reheating process

• Frosted glass: satiny look when exposed to HF

OHSiFSiOHF 242 24


v) Speciality (Cont.)

• Coated glass: unique properties metal / metal oxides Ag+ + RA Ag mirror electrically conducting with SnO2 coating (thermal SnCl4

hydrolysis)

• Photosensitive glass:– glass that changes colour upon exposure to light

Phototropic: darkens upon exposure to light and returns to original clear

sate afterwards. AgCl/AgBr


Ag+ X- Ag + X light

dark Blue-grey colorless

Non-silicate glasses are becoming increasingly important for special optical purposes,

for example in the use of glasses prepared from CaF2, AlF3 and P2O5 for infrared optics or the use of fluoride glasses for optical fibres

Heat Treating Glass

Annealing: removes internal stress caused by uneven cooling.

Tempering: puts surface of glass part into compression suppresses growth of cracks from surface scratches. sequence:

further cooled

tension

compression

compression

before cooling

hot

surface cooling

hot cooler

cooler

Result: surface crack growth is suppressed.


a) Annealing Glass

Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed.

The process may be carried out in a temperature-controlled kiln known as a Lehr.

Annealing glass is critical to its durability.

Removes internal stress caused by uneven cooling.

Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock.

If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.


b) Tempered Glass • The tempering process consists of the following steps:

1) First the glass is washed and then heated.

2) In order to temper glass, it must reach 1100°F (the softening point for glass.)

3) The glass is then cooled with cold air. Quenching with forced cold air sets up the tension and compression zones.

4) The tempered glass continues down the rollers to cool more and be packed for shipping. Glass to be tempered must be cut to size before the tempering step.

• A flow chart in the next slide provides a summary of the tempering process.

Tempered Glass: The Process


b) Tempered Glass (Cont.)

• Tempering glass:

Heat glass to softening point

Cool outside of glass quickly

Outside stiffens while inside is still hot

Shrinking inside compresses outside

Compressed outside stretches inside

• Resists fractures because surface is compressed

• Crumbles when cracked because inside is tense

Glass expands when heated

Quenching “freezes” this expansion on the outside

Center cools more slowly, and contracts. Sets up tension and compression zones.

Tempered Glass is required for door products and some windows installed near doors. If tempering is done improperly then distortion can result.

Tempered glass is stronger than annealed glass. If annealed glass (raw float) has a strength factor of “1”, tempered glass would be “4”.


What is the difference between (regular) annealed glass and tempered glass?

Annealed (regular) Glass

• Advantages:

Cost

• Limitations:

Breaks in sharp pieces

Not as strong as Tempered Glass

Size limitations

Tempered Glass

• Advantages:

4 times the stronger than annealed

Breaks into small, harmless pieces.

Qualifies as Safety Glazing

• Limitations:

Must be cut to size before tempering

Optical distortion (roller wave, strain pattern)


Examples of today’s glass products:

Containers (jars and bottles)

Flat glass (windows, vehicle glazing, mirrors, etc.)

Lighting glass (fluorescent tubes, light bulbs, etc.)

Tableware (drinking glasses, bowls, lead crystal, etc.)

Laboratory equipments (test tubes, cylinders, measuring flasks, etc.)

TV tubes and screens

Decorative glass

Fiberglass

Optical glass

Vacuum flasks


CHEMISTRY OF GLASS MANUFACTURE In general terms, soda-lime-silica glass manufacture involves melting the required

raw material mix at 1600°C, which yields a very fluid melt, from which gases can escape (especially carbon dioxide produced by the decomposition of carbonate raw materials). The glass is then worked to produce the articles required at about 1000°C, followed

by annealing at 500-600°C. Example; the float glass process, used to produce flat panes of glass suitable for

windows, illustrates this well (Fig.3).


Fig.3: Diagram of the float glass process, showing the way a continuous ribbon of glass is drawn from the melting furnace, through the float bath (which gives the perfect surface to the sheet) and then is annealed and allowed to cool before preparation for sale.

CHEMISTRY OF GLASS MANUFACTURE (Cont.)

A glass is little more than a rapidly quenched liquid The term 'Glass' can be applied to many different materials, but in common

usage it refers to quenched silicate liquids, , which behaves as a solid but retains the molecular structure of the liquid. The production of commercial glasses is therefore dictated by the

application of phase diagrams which allow the melting behaviour of particular compositions to be predicted and the optimum conditions for glass manufacture to be identified. The appropriate phase diagram is that for the system SiO2-CaO-Na2O (Fig.4).




Fig 4: Phase relation ships for part of the system SiO2-CaO-Na2O at atmospheric pressure (weight%). The system includes the following crystalline phases:

Name Formula Abbreviated formula

Cristobalite SiO2 S

Tridymite SiO2 S

Quartz SiO2 S

Pseudowollastonite CaSiO3 CS

Sodium silicate NaSiO3 NS

Sodium disilicate Na2

Si2

O5 NS

2 Sodium calcium silicate Na

4CaSi

3O

9 N2

CS3

Sodium calcium silicate Na2

Ca2

Si3

O9 NC

2S

3 Sodium calcium silicate Na

2Ca

3Si

6O

16 NC3

S6

O

Point O is the ternary eutectic, at 725ºC, with the composition 5.2% CaO , 21.3% Na2O and 73.5% SiO2

Liquidus phase relationships within the three-component system SiO2-CaO-Na2O go well beyond those relevant for glass manufacture.

Consequently, Figure 4 focuses on the silica-rich corner of the triangular diagram, as this includes most glass compositions. In this region, the silica mineral on the liquidus is cristobalite, tridymite or quartz (depending on temperature), with very steep temperature gradients particularly towards more sodic compositions. Towards the lime apex, a field of two liquids is drawn; in this field, liquid compositions separate out into two contrasting liquids, one silica-rich and one lime-rich.

These two liquids are immiscible in the same way that oil and water are immiscible, and like a good mayonnaise they are opaque to light and can be quenched to produce an opaque white solid. The other liquidus fields show shallower temperature gradients.

On the boundaries between them arrows are marked to show the "downhill direction". These all converge on a single point, where the temperature at which liquid can exist is lowest, which is a ternary eutectic. The ternary eutectic composition can be read from the compositional axes and corresponds to 5% CaO, 21% Na2O, and 74% SiO2. The minimum temperature can be read from the contours is 725°C.

In order to decide on the optimum blend of ingredients required to make a soda-lime-silica glass, the ternary liquidus diagram can be used to indicate the temperature required to initiate melting. The ternary eutectic composition is therefore the one which appears to be ideal for glass manufacture, as it will begin to melt at the lowest temperature, saving energy and manufacturing costs. Melting is carried out at 1600°C to give enough superheat to ensure that all of the solid grains within the raw materials dissolve within the liquid and to ensure that the viscosity of the liquid is sufficiently low that gases can escape. Compositions which are more silica-rich have a rapidly rising liquidus temperature, and may not completely melt, leaving a glass which contains crystals of a silica mineral or bubbles and appears frosted. It is therefore important to use this and similar diagrams not only to design batch mixes but also to diagnose problems which arise when glasses are not correctly made.

The sources of soda and lime are respectively sodium carbonate (soda ash) and limestone (dolomite is used if magnesium is needed). These materials decompose on heating with the loss of carbon dioxide. Thus, in the formulation of batches consisting primarily of silica sand, limestone and soda ash, proportions must be corrected to take into account the loss of carbon dioxide so that they correspond to the compositions required for the finished glass. In order to carry out this correction, relative atomic masses (atomic weights) are used to determine the proportions of CaO within CaCO3 and Na2O in Na2CO3:

Relative atomic masses: Ca = 12 ; O = 16 ; Na = 23 ; Ca =40

Relative molecular masses:

CaO = 56 ; Na2O = 62; CO2 = 44, CaCO3 = 100; Na2CO3 = 106

• Therefore;

100 tonnes of limestone (CaCO3) yields 56 tonnes of CaO and 44 tonnes of CO2. and

100 tonnes of soda ash (Na2CO3) yields 100 x 62/106 = 58 tonnes of soda and 42 tonnes of CO2


Glass Industries

The World Glass Industry has a gross production value totaling $82.3

billion

Fig. 14

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Recycling of Glass

• Recycle of glass is mostly used for packaging

• Recycle process


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Virtification

Definition: a new technology has been discovered to use recycle glass for radioactive waste management

Process: melt glass together with

radioactive waste in barrels or some other container

glass will then bind up with radioactive contamination into a huge glass block

radioactive waste is bond by the glass and become immobilized

keep radioactive waste from interacting with water, stop spreading the waste

Fig. 20 www.vitrification.com/ vitrification.htm


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Good & Bad of Virtification

Benefit of virtication: Prevent radioactive waste

pollution

Minimize the amount of glass waste produced

Increase the efficiency of glass use (to stabilize hazardous waste)

High volume reduction of waste

Landfill space can be saved

Volume percent of vitrified product compared to the original waste volume

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Education

Silica sand and glass industry