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Glass melting and furnaceGlass melting and furnaceGlass melting and furnaceGlass melting and furnace

by Asst. Prof. Dr. Sirirat T. Rattanachan

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Glass melting

• Batch melting, ก��������ก��� (batch) ���� ���

• Fining ก��ก���������ก��ก�ก�� ���

• Homogenizing ก��������������� ��� ���� ����ก�� ก����� �� �������!ก�"�� #����!ก��$���� �� �����%&�'(�)���ก�� ���

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Mechanism of batch melting

• ���� ��!#���*����! ����ก$�

• ก)� thermal decomposition � carbonate, sulfate, hydrate – 2Al(OH)3 � Al2O3 + 3H2O (g)

– 2H3BO3 � B2O3 + 3H2O (g)

– CaCO3 � CaO + CO2 (g) at 900ºC

– Na2SO4 � Na2O + SO2 (g) + ½ O2 (g)

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Batch melting

• ก)����� ���� �����! ก)�� ����� eutectic point – Eutectic mixture Na2CO3-CaCO3 at 775ºC– Solid state reaction

• Na2CO3 + SiO2 � Na2O.SiO2 + CO2 (g) • CaCO3 + SiO2 � CaO.SiO2 + CO2 (g)

– Formation of new eutectic mixture• Na2O. 2 SiO2 at 800ºC

– +,�ก�� �) ����� ก)�� �����ก-��– ������ ������ก�! ก)��.)ก)�)��ก�" Na2SO4

• Na2SO4 + SiO2 � Na2O.SiO2 + SO3 (g)

Note: Batch free time: ����� �*���ก������������������"(�&/

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Batch melting• ก)�ก������ ���$�� Na2O,K2O,B2O3,PbO,As2O3, Se. etc.• ก������ ���-�� �� ���กก�� ก)������ก�+��!�ก0�• Refining

– Reaction: Na2SO4 + SiO2 � Na2O.SiO2 + SO3 (g) – 2SO3 <--> 2SO2 +O2 -����(�ก�"�%&�'(�) �! ก)�ก��������ก0������!���

ก��"$� – 2As2O3 + 4KNO3 <--> 2K2O + 2As2O5 + 4NO(g)+O2 (g)– As2O5 <--> As2O3 + O2 -����(�ก�"�%&�'(�) �! ก)�ก��������ก0������!���

ก��"$� – �� ��ก1 ���2��ก��ก��� �������3���!���-���(��)������ก�� ��!4(ก�����ก$�– �� ������ก�� ���� (���%&�'(�)) ���!��� ��ก���!�!���$�

• 2SO2 + O2 � 2SO3

• As2O3 + O2 � As2O5

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Homogenizing: compositional variation (Cord)

• Melting segregation during batching melting

• Volatilization from the melt surface

• Reaction of the melt with the refractories.

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Glass furnace

• Pot furnace

• Tank furnace: 2-3 tons melted and refined in one day

• Continuous tank: feed input=feed output– Bridge-wall tanks

– No bridge walls (for window glass and plate glass) #������ 100-200 �%2 ก��� 30-35 �%2

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Viscosity of glass

#������� �����"�2)�� ���#�3��ก��

• Viscosity and crystallization

• Fining

• Viscosity and strain � annealing

• Working processes• The influence of composition (��� oxides �� �����ก��)

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Viscosity ( η)

• Viscosity is the inverse of fluidity

• Viscosity is a measure of the resistance to shear deformation with time

• Unit: Pa.s � 1 Pa.s = 10 poise , 1 poise = 1 dyn/cm2

• ���!"" c.g.s ��� ��� g/cm.s ���� poise (P)

• ���!"" SI ������� ��� kg/m.s = Ns/m2 ���� Pa.s

• 1 P = 0.1Ns/m2

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1014.5 P����������� �ก����������������������� 95% �������������� 1 � ��!"

1013 P ����������� �ก����������������������� 95% ��������� 15 ����

107.5 P �����#�$�%��������ก��&��"����'���

104 P �����#�����ก��������%���() ���

log 10 ηηηη

*���%&�'(�)�!���� working point ��! softening point ���ก��� “working range”

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Reference points

• log η = 4 -> Working point

• log η = 6 to 9 ~7.6 � Littleton point or softening point

• log η = 13.3 � Tg (Transformation temp. or upper annealing)

• log η = 14.5 � Strain point (lower annealing)

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Dilatometer

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Viscosity-temperature relation in glass melts

Glass fiber,∅ = 0.55 to 0.77 mm, l=23.5 cm,

elongates under its own wt at v = 1 cm/min

Littleton point (softening)

TL7.6

-Flow point (British)

5

Rod ∅=0.5 mm, l = 20cm, sinks into a melt

v = 1 cm/min

Forming

Sinking point (German)

TE4.22

--working point (begin of working range)----4

-Melt transferred to working area

Gob temp.G.T3

-Refining--1.5

Determination methodprocessnamesymbollog η

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Viscosity-temperature relation in glass meltsDetermination methodprocessnamesymbollog η

-Glass at room Temp.

--19

90% of stresses relax after 15 h

Lower cooling point

Strain point-14.5

90% of stresses relax after 15 min

Glass transition

Annealing point

TG13

Thermal expansion breaks down in dilatometer

Upper cooling point

dilatormetricTM11.3

--- transition from working to cooling range----10

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Viscosity-temperature relation

• The working range index (WRI) = TL-TG

• The gob temperature (GT) = 2.63 (TL-TG)+TL

• The relative machine speed (RMS) =

(TL-450)/(TL-TG+80)

• The devitrification index (DI)= TL-TG-160

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Viscosity measurement

< 104 Pa.sStokes’ falling sphere/bubble rise

Melting

105 Pa.s <η<1015.5 Pa.sFiber elongation

105 Pa.s <η<109 Pa.sPenetration viscometer

105 Pa.s <η<109 Pa.sParallel plateSoftening and

annealing

1011 Pa.s < η< 1014 Pa.sDisappearance of stress

107 Pa.s <η< 1012 Pa.sBeam bending

< 106 Pa.sMargules rotating cylinder

Viscosity valuesMethodRange

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Measurement of viscosity

• Stokes’ falling sphere/bubble rise

• Falling sphere viscometers

• η < 104 Pa.s

• A platinum ball of radius r is allowed to fall freely in the molten glass and its velocity v of fall is measured.

v

gr

9

)(2 02 ρρ

η−

=

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Rotating cylinder viscometer

• η < 106 Pa.s

• Margules: a platinum cylinder is rotated at a constant velocity in the molten glass.

• Glass velocity is calculated from the torque (M) applied to cylinder to maintain a given velocity of rotation.

torque M

locityangular ve

depthimmersion L

cylinderinner of radius R

crucible of radius R

4

)(

ic

ic

c

22

22

=

=

=

=

=

−=

ϖ

ϖπη

iccic

icc

RRL

RRM

Lic

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The penetration viscometer

• Log η = 9 to 13

• Viscosity is calculated from rate of penetration.

npenetratio ofdepth l

timeload t

force F

ball of radius R232

93

=

=

=

=

=RL

Ftη

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Fiber elongation viscometers• Log η = 8 to 14.5• Littleton softening point and annealing

• Fiber diameter= 0.55-0.75 mm, length 23.5 cm

• Heating rate 5ºc/min

areasection crossfiber A

fiber the toapplied force

elongation of rate dL/dt

lengthfiber L

)/(3

=

=

=

=

=

F

dtdLA

LFη

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Beam-bending viscometer

• η = 108 to 1013 Pa.s

load bending the tomass beam ofon contributi1.6

AL

material ofdensity

beam ofpoint -mid theof rate deflection V

beam theof inertia ofmoment theI

span L

load M

areasection cross A

6.14.2

c

3

=

=

=

=

=

=

=

+=

ρρ

ρη

ALM

VI

gL

c

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Temperature dependence of viscosity

Vogel-Fulcher-Iammann equation (VFT)

ii L

LL

TTATLTLT

ALTTB

TTLLTTLL

TLTLLLTLTLLLA

TT

BA

=

−−−−

=

−−=

−−−−−−−−−−

=

−+=

η

η

log

)(

))((

))(())((

))(())((

)(log

21

2122110

101

12322321

112232223321

0

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Example• Transformation temp. (logη= 13.3) at 550ºC

• Softening point (logη= 7.6) at 715ºC

• Working point (logη= 4.0) at 1000ºC

)8.288(

3.3830386.1log

8.288,3.3830,386.1

)1000(0.4

)715(6.7

)550(3.13

0

0

0

0

−+−=

==−=

−+=

−+=

−+=

T

TBA

T

BA

T

BA

T

BA

η

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