Ceramic – Glass

Raw materials for Glass Manufacturing:

Limestone which is present in the earthen crust across the globe is primarily formed when the remains of coal animals and plants are compacted on the ocean bottom. There are mainly two types of Limestone. The classification is based on the composition. One type is the mineral calcite which contains the calcium carbonate and the mineral type is dolomite which contains the carbonates of both calcium and magnesium. Both the above said types also contain small amounts of other minerals.

There are three distinct types of limestone which are defined by their magnesium carbonate (MgCO3) concentrations:

Dolomite limestone: This type contains about 35 to 46% of magnesium carbonate in it.
Magnesian limestone: This type contains about 5 to 35% of magnesium carbonate in it.
High Calcium limestone: This contains about less than 5% of magnesium carbonate in it.

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Ceramic – Glass

Raw materials for Glass Manufacturing:

For using in Glass manufacturing application, the limestone should essentially have about 97% of Calcium Carbonate and 0.12% of Iron oxide. The Iron oxide content required limits vary according to the type of glass to be produced. The mentioned iron oxide limit of 0.12% is for manufacturing the standard container glass. For manufacturing speciality glasses the maximum allowable lower limit of iron content is only 0.04%. This is because; iron generally provides colour shades to any material. And here in glass too Iron gives colours to glasses. So for manufacturing high quality glasses like flint or crystal type it is highly important to take care that there is not iron contamination to prevent the discoloration effect.

As mentioned in the previous blog, Limestone is used for providing the Calcium oxide source to the glass and its effect is felt mainly during the annealing stage of the glass production. But to be specific the Calcium Oxide is required in the glass for providing it strength and abrasion resistance. Calcium oxide increases the strength of the glass and its abrasion resistance capacity.

Ceramic – Glass

Raw materials for Glass Manufacturing:

Limestone crushing is the most important step in processing the mined limestone. Generally limestone is crushed by dry grinding method and wet grinding method is not preferred for limestone crushing. There are special mills available in market for crushing limestone. Earlier traditional mills like Ball mills were used for crushing limestone. But today with the advancement of technology more improved mills are available. These special mills are very efficient in producing ultra micron particle size output.

Limestone is used in glass manufacturing and another major application of limestone is in cement and mortar manufacturing. The cement is manufactured by heating limestone and clay together. Limestone is heated to form quick and slaked lime. Limestone is also used as a reagent in desulfurization process. It is also used as an additive to soil for neutralizing acidic conditions.

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Ceramic – Glass

Raw materials for Glass Manufacturing:

Next to Silica Sand and Soda Ash, Limestone is the major raw material used for manufacturing glass. About 14 - 18% of the raw material batch weight consists of limestone. Limestone is nothing but Calcium Carbonate (CaCO₃). The limestone is added to the batch to provide the Calcium oxide for glass formation. The limestone when heated gets converted to Calcium oxide (CaO). That is the Calcium Carbonate releases Carbon dioxide on heating and thereby forms Calcium oxide.
CaCO₃ (on heating) ————> CaO + CO₂

The limestone is commonly referred to as Carbonate Rock. Also the mineral dolomite falls in the carbonate rock variety. Limestone is sedimentary rock category and is mined from the earthen crust. Both open pit and underground mining is popular for mining limestone. There is not much processing required to the natural limestone. Generally the mined limestone mineral is first sawed for removing impurities and then crushed in Grinding Mills followed by screening to check uniform size distribution. The size of the mineral limestone varies from application to application.

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Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process:

Ammonia recovery yield is controlled according to the permitted ammonia concentration in the released liquid. The lower the permitted value, the higher the quantity of stripping steam and therefore the global energy consumption, and the higher the cost of the ammonia recovery. This control can only be applied to a theoretical minimum ammonia level. After cooling and condensation of steam, the gaseous phase containing recovered CO2 and NH3 is returned to the absorption area for reuse.

The liquid phase coming out from distillation unit contains: unreacted sodium chloride, calcium chloride resulting from reaction with NH4Cl, solid matter that is derived primarily from the original limestone and finally, small quantity in excess of lime that can ensure a total decomposition of NH4Cl. This liquid called “DS-liquid” or “Distiller Blow Off“will be treated in different ways depending on the particular site and processes used.

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Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process:

One of the major achievements of the Solvay process is the high efficiency of the ammonia recycle loop. This loop circulates roughly 500 to 550 kg NH3/t soda ash from which the ammonia loss is less than 0.5 % of this flow rate. The purpose of this important process “distillation” is to recover ammonia from the ammonium chloride containing mother liquors recovered from the bicarbonate filters/centrifuges.

After pre-heating with outlet gas from the distiller, supported by the injection of steam at the bottom of the NH3 stripping column, the mother liquor releases almost all its CO2 content. Addition of alkali normally in the form of milk of lime decomposes NH4Cl into NH3 which is stripped from the solution by injected low pressure steam at the bottom of the distillation column. The outlet solution contains calcium chloride together with all the residual solid materials.

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Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process:

In some cases a small part of this “crude” bicarbonate, which although predominantly sodium bicarbonate also contains a mixture of different salts (ammonium bicarbonate, sodium carbonate and sodium chloride), may be extracted from the Solvay process cycle to be dried as “crude” bicarbonate product made without purification, by simple drying process. This crude product may find applications in some commercial outlets. Sodium bicarbonate cake is heated (160 to 230°C) to achieve calcination into a solid phase «light soda ash» and a gaseous phase containing CO2, NH3 and H2O. This gas is cooled to condense water and the condensates formed are sent to distillation for NH3 recovery, either directly or via filter wash water. After cleaning, the gas (high CO2 concentration) is compressed and sent back to the carbonation columns. Normally, energy needed for sodium bicarbonate calcination is provided by steam that condenses in a tubular heat exchanger which rotates through the sodium bicarbonate. The method consisting of heating externally by gas or fuel oil combustion in a rotating drum containing sodium bicarbonate is occasionally encountered.

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Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Sodium bicarbonate Formation :

Separation of sodium bicarbonate crystals from mother liquor is achieved by means of centrifuges or vacuum filters. After washing of the cake to eliminate mother liquor chloride, it is sent to calcination. The liquid phase “mother liquor” is sent to the distillation sector for ammonia recovery. Where filters are used, air is pulled through the cake by means of vacuum pumps. Thereafter, this gas carrying ammonia and some CO2 is cleaned by a washer fed with purified brine before exhausting to atmosphere. “Crude” sodium bicarbonate manufactured by the carbonation process is the primary “output” of the Solvay ammonia soda process. The bicarbonate produced in this way is the feed to the calcination stage described, for the conversion to the finished product solid soda ash.

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Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Sodium bicarbonate Formation :

Precipitation: As mentioned in the ammonia absorption step this is a closed loop process. The purified brine is allowed to react with the waste gas ammonia to form the ammoniacal brine. This ammoniacal brine is then allowed to react with excess carbon dioxide and this reaction results in the formation of Sodium bicarbonate. The formed sodium bicarbonate is precipitated as crystals by addition of water. This carbonation process is carried out in carbonator towers. The addition of CO₂ with ammoniacal brine will release ammonia gas. Thus the ammonia gas formed in the carbonation towers are cooled and recycled back to the system. This ensures that there is no ammonia gas outlet to environment and also is used for reacting with purified brine to form the ammoniacal brine.

Calcination: The precipitated sodium bicarbonate crystals are then heated (calcined) to a temperature of about 160 to 230^oC. The calcination of sodium bicarbonate cake will produce light soda ash and will release gases like CO₂, NH₃ and H₂O.

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Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Ammonia Absorption :

This is a closed loop process. Closed looping is necessary to ensure that no free ammonia is let out to the atmosphere as such. And it is a highly important step to prevent environmental pollution. This step is the initial processing step in the Solvay’s process after the brine purification. The purified Brine is made to react with ammonia to form the ammoniated brine.

The ammonia that is reacted with the brine is not the fresh one… This ammonia is the recycled gas from the carbonation towers. Ammonia is recovered by recycling the outlet gas from the distillation plant to the absorption stage where it is absorbed in purified brine. This flow mainly contains recovered NH3 and a quantity of CO2. This chemical operation is achieved in equipment that allows close gas/liquid contact. As this is an exothermic reaction, cooling of the liquid is necessary during the operation to maintain efficiency. The outlet solution, with a controlled ammonia concentration, is called ammoniacal brine. Any gas that is not absorbed is sent to washer contacted with purified brine to remove traces of ammonia before it is recycled or released to the atmosphere.

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