Glass
Introduction

Glass is often defined as an inorganic product of fusion that has cooled without crystallizing.The glasses are a familiar group of ceramics; containers, lenses, and fiberglass represent typical applications. As already mentioned, they are noncrystalline silicates containing other oxides, notably CaO, Na2O, K2O, and Al2O3, which influence the glass properties.
  1. According glass basic principles of physics,the glass is hard, brittle, amorphous,transparent,super cooled liquid having high viscosity.
  2. Chemically glass can be defined as a completely vitrified solid solution of oxides,formed by rapid cooling a previously molten mass of the salts.
  3. It is not a true solid as it does not posses sharp melting point, Definite formula or Crystalline structure.

Within certain limits a glass may be represent as:
xR2.O,yMO.6SiO2
R = Atom of monovalent alkali metals like Na,K etc.
M = Atom of Bivalent metal like Ca,Pd,Zn etc.
PROPERTIES OF VARIOUS RAW MATERIALS

SILICA SAND (SiO2):
The basic ingredient of most industrial glasses is sand, consisting of 95-99 % Silica (SiO2), this is a network former. Silica makes up 75% of the earth's crust. This relatively inexpensive element still has to meet precise specifications in terms both of Granulometric, Chemistry and Mineralogy. A characteristic of Silica is that it is difficult to melt and the melting temperature of the sand is 1723 to 1728℃.

SODA ASH (Na2CO3):
Soda ash is Source of Na2O.Soda ash used in the glass manufacturing to reduce the melting temperature of silica sand as a flux. Soda ash acts as a strong flux, and therefore greater is the amount of soda ash, the readier is the melting. Glasses containing high amount of Sodium oxide are soft and can be easily made in to shape. The increase of Sodium oxide in the glass increases the viscosity range, the co-efficient of expansion and reduces the tensile strength, annealing temperature and the durability. The maximum limit of Soda ash in Glass is 18 % and above this the glass is liable to be detoriated by weathering and is liable to become dull and dirty in appearance after some time.
Soda Ash works as a Fluxing agent & it’s a Network Modifier.
The Melting temperature of Soda Ash is 851℃. The decomposition reaction can be written as follows……

Na2CO3 (850℃) → Na2O + CO2

DOLOMITE (CaCO3 MgCO3):
Dolomite is a sedimentary rock and it is mined by exploding rocks. Dolomite is a mixed mineral consisting of calcium & magnesium oxide. Magnesium oxide is similar to lime stone but does not have the same tendency for devetrification & helps to counter the former’s negative effects. Primary supply to batch is magnesium & supply of lime is secondary. Magnesium oxide in small quantities in glass gives a rapid melting rate and improves the working properties of lime glass and prevents the tendency to devetrification and lowers the annealing temperature.
The decomposition reaction can be written as follows……
CaCO3 MgCO3(750℃) → MgO + CaO + CO2

CALCITE/ LIME STONE (CaCO3):
Calcite is a source of CaO. Calcium oxide imparts a very great chemical stability to the glass. High calcium oxide in glass tends to devitrify (Devitrification occurs when the molten viscous liquid crystallizes leaving defects in the glass), and form Wollastonite. Increase of calcium oxide increases the heat conductivity, annealing temperature, mechanical strength, thermal endurance and the durability of a glass and it lowers the coefficient of expansion.
The decomposition reaction can be written as follows……

CaCO3(898℃) → CaO + CO2

SODIUM SULPHATE (Na2SO4):
Sodium sulphate also called as Salt cake. Salt cake is a synthetic raw material & most important fining agent for glass manufacturing (as a fining agent as sulphur dioxide is liberated at a later stage in the fusion.). Melting temp of Sodium sulphate is 885OC. Sodium sulphate is hydrophilic like soda ash below 32.4℃. As a result
the batch temperature must always be kept above this temperature in order to prevent dust formation. The reaction between salt cake and silica sand occurs at about 1500oC. It is found that reaction between sodium sulphate Na2SO3 and silica takes place much readily. Hence some reducing agent such as Coal is added to the batch containing salt cake to reduce it to Sulphate.

Na2SO4 1300℃ → Na2O + CO2
Na2SO4 + C → Na2SO3 + CO

Practically about 2.5% of Coke to the salt cake is usually added to the batch. Salt cake readily absorbs the moisture and forms hydrated Gabber’s Salt, Na2SO4 10H2O, which on exposure to atmosphere forms hard lumps difficult to grind. It should be stored in a dry place.

COAL OR COKE:
Reducing agents promote the incorporation of some batch forming oxide in a glass. Carbon as coal is employed with Salt cake to reduce it to sulphate, so that the reaction with silicate may take place at a lower temperature.

CULLET:
Cullet is broken glassware or waste glass and is used as an auxiliary batch ingredient in glass manufacturing. The addition of cullet in a batch facilitates the melting and fining of the glass. The usual amount of cullet added is 18 to 25% of the batch.
There are two types of Cullet being used

  1. Domestic cullet
  2. External cullet
Domestic cullet having different type
  1. Normal cullet
  2. Reflectasol cullet
  3. Mirror cullet
  4. Coater cullet
External cullet is being received from the dealers and suppliers.

Advantages of using Cullet:

BATCH PLANT

In the batch plant the various raw materials are used for making glass they are received, and in stored in separate silos. It is the place where raw materials are required to make clear or color glass are weighed, mixed and conveyed to furnace.

BATCH PLANT – EQUIPMENT OVER VIEW
SILOS:
Silo is used to store various types of raw materials. The silos are made of concrete. Expect the soda ash silo. It is made of mild steel because soda ash is hygroscopic nature. It wills absorbs moisture and forms lumps.

CONVEYORS:
Conveyors are used to transport the materials from one place to another place.


FURNACE SPECIFICATIONS:- Name - F350FF
Type - Cross fired regenerative type Furnace fuel - Heavy Furnace Oil
No. of ports - 7
Reversal cycle - 20 minutes
Depth of glass in Melter - 1424 mm
Total holding capacity - 1710 MT

GENERAL DESCRIPTION:

FURNACE SPECIFICATIONS:
Name - F 470FL Type - Cross fired regenerative type furnace Fuel - Heavy Furnace Oil No. of ports - 7 Reversal cycle - 20 minutes Depth of glass in melter - 1450 mm Total holding capacity - 2084 MT GENERAL DESCRIPTION:
Furnace is a closed chamber made up of different types of refractory. Heat is produced inside by the combustion of fuels which is utilized to melt the batch. In a glass melting furnace different sections and their functions are as given below.
SECTION FUNCTION
MELTER Batch gets melted and volatile gases escapes.
NECK Narrow section with a physical partition through which glass enters next section.
WORKING END Glass temperature brought down. Homogenization of glass takes place by convectional currents.
CANAL
(SG and PB canals) Glass temperature lowered in a controlled manner before formation to avoid thermal distortion.

FURNACE MAIN PARTS:
The furnace is divided into five regions. They are
  1. Dog house
  2. Melter
  3. Neck
  4. Working end
  5. Canal
1. DOG HOUSE
It receives the raw materials from the batch charger. Water cooled dog house cooler is there to prevent the escaping of hot furnace gases.
BACK WALL:
Back wall is the suspended wall present at the beginning of the melter region. Before the back wall is the Dog house cooler. The purpose of the dog house cooler is to prevent the loss of heat from the melter region. Back wall is made up of silica refractories which are suspended form the steel structure. Metallic shield enclosure is provided over the back wall and cooling air is circulated at seven inlet ducts to prevent the heat loss and also to prevent the damage of the back wall. The air circulated for cooling the back wall must be at higher pressure when compared to the melter pressure. Higher pressure on the back wall side is kept by adjusting the out let dampers. The purpose of the back wall is
2. MELTER
Melter is the region where the crystalline solid batch materials melts and forms the molten glass. For melting this batch materials two sources of energy are employed one is the Fuel oil and the other is the Electrode Boosting. Heat transfer form the Fuel oil is by radiation and from the Electrode is by Convection. The melter constitutes seven ports on each side and holds the flame above the molten mass. All the Ports have five burner slots. From the 1st to 6th port firing is done on 1st, 3rd and 5th position and in 7th port firing is done in 1st position. For first twenty minutes firing is done from right side port and flue gases move to chimney through left side regenerator. For the next twenty seconds no firing is done.
Again for twenty minutes firing is done from left side and flue gases move to chimney through right side regenerator. Secondary air picks up heat from the regenerator blocks and is used for the combustion of the fuel oil. The oil at 15 bar pressure is charged through a nozzle. Gets atomized mechanically and mixes with the primary air at 1 to 0.5 bars before it gets fired. Swirler is a small circular metal plate having small slots in an inclined manner and it pushes the fuel oil flow in to the furnace to help in easy and better atomization. Ideally the flame length should be 2/3rd to 3/4th of the width of the furnace. When the fuel is over atomized, the flame length becomes short and the invisible part of radiation emits UV rays and heats the regenerator bricks. When the fuel oil is less atomized, the flame length is long and the invisible radiation is in IR range and heat up to the bottom layer of glass. Primary air enters the root of the flame where the temperature is high and results in more NOX production. To minimize the entry of primary air, a ceramic ring called the burner block ring is introduced in the burner block.
As the age of the furnace increases its efficiency gets reduced. Currently the furnace operates at around 100% efficiency. The fuel consumed per hour stands at around 4850kg for 800 to 850MT pull at 20% cullet. The net firing time is for 23.5 hours and time between reversals consumes 0.5 hour.
The melter is 37.5 m long. It melts the raw powder to molten glass. Dam wall is present at dog house. It is made of AZS. Dam wall separates the current M2 & M3. It prevents the impurities from carry over in to the Working end and acts as a physical barrier.
Shadow wall is present just before the neck region to prevent the flow of working end gases into the melter. The gap between the shadow wall and glass level is maintained. It is made of silica bricks supported by iron plate. It is cooled by air from shadow wall blower.
The shadow wall cooling air pressure is also maintained at higher pressure when compared to the melter pressure.

3. NECK
Neck is 9.43m long. It’s tapered upwards in a slant manner to avoid deposition of defective glass at the corners. Barrage is put in the neck region from both left and right side of the furnace. Barrage is a rectangular box whose inner sides are cooled by cooling water. The purpose of barrage is to divert some portion of M3 current back towards the melter and there by increasing the homogenization. In this way the temperature difference between the surface and bottom of glass are reduced. Barrage also eliminates certain surface defects. The size of barrage employed for clear glass production. Barrage is positioned in such a way that some distance is above the surface of the glass. The gap between the two barrage elements is maintained.

4. WORKING END
The working end is 17.5m long. In working end the glass temperature is reduced to about 1200-50oC by using two air ducts through which air is blown in counter direction of flow of glass. The pressure at working end is 3.5 - 4 Pascals. The Working end pressure is controlled by the chimney damper. Feeder drain points are present in the Working end exit having a capacity of 150 MT/day to have uninterrupted furnace pull. Furnace glass level measurement both Manual and Automatic is present in the Working end which is as important furnace control parameter.

5. CANAL
Canal delivers the melting glass to the float bath with a temperature as constant as possible. The canal is provided with equipment to homogenize the glass. The canal has a converging end with a Spout lip from which the glass flows on to the float bath. The canal bottom, a key zone of the process is made of Zargal -M, a refractory that does not cause any defects. The canal superstructure is closed off by crowns separated by Curtains. The side walls of the canal is made of sillimanite bricks. They support the installation of Gas burners when needed. The canal is 6.5m long and is tapered towards the spout lip. The purpose of canal is to maintain laminar glass flow before being poured into the float bath through spout lip and also to minimize the temperature difference between the top surface and sides. The thermal homogenization of glass is obtained using AG3. The AG3 are slow running agitators. The arms are made of mullite refractory and covered with platinum sheet. The curtains available above the glass level to maintain the canal pressure. The pressure at canal is maintained. The PB temperature is measured above the glass level at the end of canal and is maintained at 1100 o for clear glass.

IMPORTANT PARAMETERS:
Dog house, hotspot, Working End and shadow wall temperatures are measured using optical pyrometer at the specified locations. B5 and PB temperatures were measured using thermocouples installed in the furnace. The temperature is controlled by adjusting fuel flow. The fuel flow to be adjusted to the individual ports. If the furnace pressure goes below atmospheric then cold air and dust enters into the furnace and defects get generated. It also decreases the temperatures inside.
If the pressure is high at working end, condensation of sulphur takes place at the crown and on moving down creates more bubbles.
If the pressure is high at the canal correspondingly the PB temperature gets affected and the glass may spread into more area on the float bath and may touch the sides.
Furnace pressure at the working end is controlled by the damper adjustments and also by changing the rpm of blowers. Furnace pressure at the melter is controlled by adjusting the butterfly damper present in the main flue gas path in remote mode. There exists a low clamp provision in the DCS so that the butterfly damper is not closed fully at any moment by fault. The stack damper at the downstream of butterfly damper is kept at a constant opening.
The glass level is another critical parameter to be maintained inside the furnace. This level can be altered by changing the rpm of the batch charger motor. The fluctuation in the glass level can lead to wash out of the side wall and more defect formation.

REFRACTORIES:
The refractory used in the glass contact areas of melter, working end and neck is fused as AZS (side wall and bottom). The breast wall is made up of AZS/ alumina in the high temperature areas and Silica refractories in the low temperature areas. The crown is made up of silica bricks. The canal is made up of alumina bricks. In the regenerator, the bottom layer is made up of sillimanite bricks, above which there are checker blocks of different grades. The regenerator crown is made up of silica bricks. ELECTRODES:
Bottom of the molten glass is heated by electrodes. There exists 19 electrodes made of molybdenum and are grouped into three segments.

Fusion-1 -- 6 Electrodes
Fusion-2 -- 6 Electrodes
Source -- 7 Electrodes
Total -- 19 Electrodes

The electrical connectivity is through glass itself .Fusion-1, fusion-2 & source are inserted into the glass. Maximum energy is transferred through the tip of the electrode. The electrodes get consumed with time at the rate of 2mm per month. So the electrodes are pushed in once in six months by 5mm.
Now all electrodes are extended by 0.5 m. around electrodes, cooling and inert gas is provided because molybdenum above 500OC reacts with oxygen and forms molybdenum oxide which is powder and breaks. Electrodes are surrounded by KANTHAL (alloy) spool. The Electrode is surrounded by water jacket and nitrogen is allowed in between water jacket and electrode and a positive pressure is maintained. Ceramic gasket is put in between water jacket and KANTHAL for sealing purpose. All electrodes were mounted on a heavy duty jack provided with wooden insulation and the entire area is fenced.
REFINING
Disappearance of all gases likes CO2, SO2, and O2. This phenomenon occurs simultaneously with melting. The mechanism is as follows. The partial pressure difference of gases between the bubbles act as a potential for the coalition of the bubbles to form a bigger bubble which comes to the top easily and collapse.
HOMOGENIZATION
To make the same physical (example density and refractive index) and chemical properties of the entire melt by establishing continuous convection currents. It follows above two. Conditioning: To bring down the temperature of the glass slowly to the workable condition without affecting its homogeneity. It corresponds to a viscosity. The batch is fed to the furnace in an intermittent manner in order to get a batch pile in a more or less sinusoidal form inside the melter .This helps to increase the heat transfer area to the batch.
BATCH LINE
More batches are melted in the center of the furnace compared to the sides giving a V shape to the batch .This happens due to following reasons
  1. Temperature is maximum at the center as the flame covers almost 3/4th of the furnace width.
  2. Movement of glass is less along the sides due to viscosity effect.

FOAM LINE
The line that differentiates between the refining region and after refining region is the foam line after which the glass is completely molten.

TRANSVERSE CONVECTION CURRENT (M1):
The current direction is from furnace towards walls. These currents promote batch melting in melting zone and homogenizing in refining zone. It is observed in the V of the batch charging zone.

LONGITUDINAL CONVECTION CURRENT (M2):
The current direction is from batch charging to canal. The denser cold glass flows towards the bottom, then towards the hot point, thus facilitating the melting process.
  1. Batch charging to Hot point
  2. Hot point to Canal

These currents promote better homogenizing, correct refining and conditioning.
SURFACE CURRENT (M3):
The current flows across the whole furnace length from Batch charging zone to Canal. The objective is to prevent unmelted glass flowing from the Batch charging zone directly in to the Working end and Canal, as this would cause defects or ribbon breaking in Float.
Positive pressure of ±1Pa above atmospheric pressure is maintained in the melter section to avoid the ambient air coming inside. The back wall prevents the escape of flue gases from the furnace to the surroundings. The working end pressure is more than the melter pressure (3Pa) to avoid the gas flow from the melter to the W.E., the shadow wall in the neck also helps in this function. The working end pressure is maintained by controlling the air flow through the blowers and the gas flow through the WE stack. . The dam wall at the bottom of the melter helps in the formation of currents in the furnace.

FURNACE EQUIPMENTS,

INJECTOR
Injects the pressurized fuel along with the atomizing air. The fuel is preheated and pressurized in the fuel preparation room before coming to the injector. Preheating is done by steam heat exchangers.

FIRING REVERSAL SYSTEM

During every 20 minutes reversal of firing takes place. During this time secondary air comes through the firing side and the flue gases goes to the chimney through the other side of the regenerator. Checker blocks in the regenerator absorb the heat in the flue gases and supplies it to the secondary air during the next reversal.

BUBBLERS

They are equipments used for refining of the melt. Nitrogen gas is passed through the bottom of the melt so that they help in refining as explained. There is provision for 16 bubblers. The bubbler also generates localized currents and also its increase M2 current.

TEMPERATURE MEASURING EQUIPMENTS
  1. Optical Pyrometers
  2. Thermocouples


FEEDER :
Feeders used to take out the molten glass in order to maintain the furnace pull during low thickness run. Feeder drained point is placed at the exit of working end on the both sides.
The Feeders are operated under following conditions:
  1. 2.5 mm glass productions
  2. 3.5mm, 4mm reflectasol production
  3. Under float shutdown emergency


BLOWERS
Side wall cooling is done just below the glass level to avoid corrosion. Corrosion is maximum in this region due to the following reasons.
  1. The cross convectional currents in the furnace
  2. Maximum reactivity in the solid-liquid-glass interface

Barrage: water coolers used in neck for effective homogenization

AGITATORS
Equipment used for physical homogenization by cutting down the reams formed during the cooling .This has got fingers which rotate in the melt.

CURTAINS
They separate the atmosphere in the working end and the atmosphere in the canal. The curtains also limit disturbances when homogenizing equipment is being introduced.

BURNERS
They provide energy boosting to avoid glass settling in the canal during a shut down.

VENTILATION
A system is provided to ventilate the canal bottom. The cold air is used to cool the canal bottom.

PRESSURE MEASUREMENT
It is essential for furnace control and to make quality glass. Pressure is measured in quiet zones to avoid subjecting measurements to variations. The atmospheric pressure is measured and taken as reference. The pressures in the furnace are then compared to this reference pressure. The objective is to maintain the pressure in the canal at a higher level than atmospheric pressure.
Pressures are measured on either side of the furnace. The DCS calculates an average value to minimize variations. The canal pressure does not have a specific control. However, the canal pressure is related to the working end pressure which is controlled. When a measuring apparatus is defective, this can be detected by the drift in its results compared with other measuring devices.

TEMPERATURE MEASUREMENTS
To control the flowing glass temperature, the canal is equipped with an:
  1. Optical Pyrometer and a PB Thermocouple
  2. Blind thermocouples on the bottom

REACTIONS INSIDE THE FURNACE

DEGASSING REACTIONS

Na2CO3.H2O 105℃→ Na2CO3 + H2O

CaMg(CO3)2 740℃→ CaO + MgO + 2H2O

CaCO3 880℃→ CaO + CO2

REACTIONS IN SOLID STATE

Na2CO3 + CaCO3 600℃~810℃→ Na2Ca(CO3)2 (Eutectic)

2SiO2 + Na,Ca(CO3) 700℃→ Na2O.2SiO2 + CaO + 2CO2

SiO2 + Na2,Ca(CO3)2 850℃→ Na2O.3CaO.6SiO2( FOR SiO2 < 50 Micro Mts.) + CO2 Na2O.2CaO.3SiO2( FOR SiO2 > 50 Micro Mts.)+ CO2

CaO + SiO2 850℃→ 2CaO.SiO2

xNa2CO3 + y SiO2 851℃→ xNa2O.ySiO2 + XCO2

REACTIONS IN LIQUID STATE

Na2O.2SiO2 + 2CaO.SiO2 700~850℃→ Na20.3CaO.6SiO2 + Na2O.2CaO.3SiO2

Na2O.2SiO2 + Na2O.3CaO.6SiO2 + Na2O.2CaO.3SiO2 740℃→ Ternary Peritectic

Ternary Paratactic 850℃→ Vitreous Phase + SiO2(s)

REFINING WITH SALT CAKE

Na2SO4 + 2C 700℃→ Na2S + 2CO2

3Na2SO2 + nSiO2 + Na2S 1100℃→ (Na2O)4(SiO2)n + 4SO2

Na2SO2 + nSiO2 1200℃→ Na2O.(SiO2)n + SO2 + ½ O2

2Na2SO4( Excess )* 1400℃→ 2Na2O + 2SO2 + Oz