Ceramic – Glass

Raw materials for Glass Manufacturing:

Hydrated lime is used in the following applications:

• Water and waste Treatment
• Flue gas desulfurization
• Agricultural application used for soil stabilization
• Used in building products like mortars and stuccos
• Used in plasters

There are two types of hydrated lime. They are Type N and Type S hydrated lime. This product difference is in the applied pressure level for forming the hydrated lime. This is because the calcium oxide can quickly hydrate at atmospheric pressure. Whereas the hydration of Magnesium oxide hydration requires a long soak period and/or high pressure levels to completely hydrate. The N type hydrated lime is produced at atmospheric pressure. This results in a calcium hydroxide/magnesium oxide product which has high chemical availability. The S type hydrated lime is produced at elevated pressures to completely hydrate the calcium and magnesium compounds.

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Ceramic – Glass

Raw materials for Glass Manufacturing:

There is a by product generated in the limestone calcination process. Process generates Lime kiln dust. It can be either in wet or dry form (wet or dry) and mostly consists of partially calcined quicklime and coal fly ash. This by-product also has variety of applications which includes environmental treatments, as a raw material for cement production, soil stabilization and as a neutralizing agent in agricultural applications.

Another product of limestone is the hydrated lime which is produced from calcined limestone (i.e. quick lime). Limestone when calcined forms quicklime and this quicklime when added with water can be converted into hydrated lime. The addition of water has to take place in controlled conditions for converting quicklime into a fine and dry powder of hydrated lime. The Hydrated lime is used in many of the applications where quicklime is used. The hydrated lime is nothing but diluted form of quick lime and so it can be used in most of the applications which uses quick lime.

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Ceramic – Glass

Raw materials for Glass Manufacturing:

Having seen the basic varieties of limestone (Calcium Carbonate) we can now discuss the products of limestone and how they are manufactured. One of the most widely used lime product is quick lime. This is nothing but calcined variety of limestone. The limestone is calcined in kilns to form quick lime which consists of calcium and magnesium oxides. The quicklime has a wide variety of environmental and industrial applications. Major uses of quicklime are the following

• Flue gas Desulfurization (FGD)
• As a steel fluxing agent
• Production of Fiberglass, Pulp and Paper, Gold, Aluminium etc.

The process of converting limestone to quicklime is done through calcination in energy efficient rotary or vertical kilns. The operating temperature of these kilns is greater than 2000 °F to allow the limestone calcination process.

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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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