Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Lime Calcination :

Particle size distribution of the limestone from quarries is generally between 40 and 200 mm. The more homogeneous it is, the better the lime kiln will work but the greater the amount of limestone fine by-product produced at the quarry. Coke, and rarely coal, is used in lime kilns for soda ash production due to the necessity to obtain the highest CO2 concentration. Other type of fuels, natural gas or fuel oil, would result in a too low CO2 concentration in the kiln gas. This is important because the kiln gas is used further in the process for its CO2 contents. Higher CO2 concentration enables reduction of the equipment size and ammonia losses. The particle size distribution of the solid fuel has to be appropriate in order to get a homogeneous distribution within the kiln. The storage of coke requires no specific precaution other than normally adopted, i.e. open ground storage.

To be contd…

Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Lime Calcination :

Limestone is a naturally occurring mineral that can be mined and ground to meet the process requirement. The naturally occurring deposits are originated from calcareous shell and skeletons of biota as well as chemical and bio-chemical precipitation in the sea during geological ages. For best utilisation of the calcination process and to obtain maximum production efficiency it is highly essential that the content of Calcium carbonate in the limestone is very high. A high CaCO₃ content ensures that there is very little difficulty in the limestone calcination process. Also one point to note in such limestone is that there will be cost savings in the form of transport as high CaCO₃ content means little inert part in limestone and so the transport cost in transferring inert material can be minimized to a considerable extent. A limestone rich in CaCO3 will not only reduce solid matters in the effluent of the distillation unit but will also, for those soda ash plants that have settling, reduce the volume of solids to be treated.

To be Contd….

Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Lime Calcination :

Lime calcination process produces raw burnt lime and approximate content of CaO in this raw calcined lime is about 75 to 90 percent. However this raw lime cannot be used as such in the Solvay’s process. This is because it is highly difficult to maintain a constant feed rate of CaO in the input lime as it varies highly across the feed. So the lime has to e converted into milk of lime for usage to ensure that the CaO content in the feed is constant. By hydration of CaO into milk of lime a better control of the alkali addition can be achieved during the ammonia recovery step.
Hydration of the raw burnt lime is carried out in slakers where raw burnt lime and water flows are fully controlled to ensure that the alkali content of milk of lime produced is as constant as possible. This reaction is a highly exothermic. A part of the heat generated vaporizes some water which is released from the slaker vent. During the hydration, fine inert materials contained in limestone (sulfates, silica, clay, silico-alumina compounds, unburned limestone and others) can mainly be found in milk of lime. Larger particles are separated by screening, then washed and recycled or released out of the process. The unburned pieces of limestone are recycled.

To be Contd….

Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Lime Calcination :

In the lime kiln operation, we should consider 2 factors as important; one being the quantity of gas and another composition of gas. The operational capacity of the kiln is purely determined by the amount of lime required for ammonia recovery in the distillation stage and hence the quantity of CO₂ required will not be considered a driving factor for kiln operational capacity determination. However the kiln often produces only excess of CO₂ than the process demand and hence this excess gas is released as waste to air most of the time. However it is essential to de-dust the gas before emitting it to air and it has to be noted that the gas emitted will be similar to that in demand in the process but as this is an excess amount it has to be considered a process waste. Measures should be taken to see to that the fuel flow rate is correct enough to minimise the emission of carbon monoxide. The CO emission depends upon the load, the quality of fuel used and the composition of the limestone used as raw material.

To be Contd….

Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Lime Calcination :
As discussed in the earlier blog, the lime calcination kiln used in Solvay’s process is different from the other normal kilns used for calcination as this kiln has to consider the production of outlet gases with maximum amount of CO₂ in them. The CO₂ from the CaCO₃ is used for forming the sodium carbonate molecule. This excess CO₂ for the bicarbonate production is derived from the energy source. Also the process does not demand highly pure lime as required by the other lime industries. In Solvay’s process, the limestone is calcined in a temperature range of 950 to 1100^oC in vertical shaft kilns. The retention time of lime in the lime calcination kiln is about 24 to 48 hours. In certain plants the limestone is screened through sieves before kiln loading in order to improve particle size distribution of the raw material. The kiln generally uses solid fuel than fuel in liquid or gaseous form.

To be Contd….

Ceramic – Glass

Raw materials for Glass Manufacturing:
Solvay’s Process – Lime Calcination :

Lime Calcination is a multi-purpose step in the Solvay’s process. This step is essential to produce milk of lime production and equally important in generating excess Carbon dioxide that is used in the carbonation process. So while selecting a kiln for lime calcination we should keep in minds that the kiln should be able to generate maximum amount of carbon dioxide. Generally for lime calcination rotary, vertical shaft or annular kilns are used. Of these types the best suitable for the soda ash Solvay’s process is Vertical Shaft kiln as this kiln can produce about 35 to 40% of Carbon dioxide where as the other kilns can generate only about 25% of carbon dioxide. Also only the vertical shaft kiln only can give maximum thermal efficiency when compared to the other kilns. Also other kilns need a highly quality limestone as their design demands raw material of uniform and small particle size. However the vertical shaft kiln can handle a wide particle size distribution and also has the design advantage of providing reserve gas capacity of several hours without loss of kiln control.

To be Contd….

Ceramic – Glass

Raw materials for Glass Manufacturing:

Solvay’s Process – Brine Purification :

The alkaline reagent can be lime milk or sodium hydroxide. Usually milk of lime is used as the reagent because this is produced in abundant for ammonia recovery. Process should consider sufficient settling time for letting the precipitation process to happen and let settling process happen in brine settler or in a decanter. So the output of this step is nothing but purified brine and precipitated crystals of Magnesium hydroxide and calcium carbonate and little amount of Gypsum. These solids which are not useful in the process can be treated separately for making them as useful process by-products or also can be let out as process wastes which are disposed of with the other wastes generated during process. Several unsuccessful attempts had been made to remove the calcium carbonate content in the brine purified waste. This is not achievable due to the chloride presence after the effluent treatment and also the effluent treatment process could not fully purify the CaCO₃ from process impurities.

To be Contd….

Ceramic – Glass

Raw materials for Glass Manufacturing:

Solvay’s Process – Brine Purification :
The Brine is one of the most important raw materials in the Solvay’s process. As mentioned plants which are situated near seas can use the sea water as Brine after processing for purification and other plants which are far away from sea should use the common salt mixed in water as Brine again after purification. Purification of Brine is done to remove impurities like calcium and magnesium. Calcium ions are made to react with Sodium Carbonate and are precipitated as calcium carbonate. As Calcium Carbonate is insoluble it can be removed easily. Process can be modified to precipitate certain amount of calcium as gypsum (CaSO₄.2H₂O) by including sulphates in the process. Similarly Magnesium ions are removed as precipitates of Magnesium hydroxide. To precipitate the Magnesium ions, we should add an alkaline reagent.

To be Contd….

Raw materials for Glass Manufacturing:

Solvay’s Process: The soda ash formed as a result of the process explained in the last blog is of light dense type. To convert that light soda ash into dense type, densification process has to be followed. In Densification process water is added to the Sodium Carbonate and as a result sodium monohydrate is formed

Na₂CO₃ + H₂O — > Na₂CO₃.H₂O

This process is then followed by drying where the water is liberated to give dense soda ash

Na₂CO₃.H₂O — > Na₂CO₃ + H₂O

So considering the above said process, the whole production process of soda ash can be divided into

• Brine Purification
• Lime calcination and milk of lime production
• Ammonia Absorption
• Sodium bicarbonate Precipitation
• Separation of Sodium bicarbonate from mother liquor
• Calcination of Sodium Bicarbonate
• Ammonia Recovery
• Product Storage and handling

We shall see in detail each of the above said process steps in the coming blogs.

To be Contd….

Ceramic – Glass

Raw materials for Glass Manufacturing:

Solvay’s Process: Practically following are the steps followed in soda ash production in the solvay’s process.

• The Brine (i.e. Salt Solution) is allowed to react with ammonia to form ammoniated brine.

  NaCl + H₂O + NH₃ — > NaCl + NH₄OH

• The ammoniated Brine is allowed to react with carbon dioxide to form ammonium Carbonate

  2 NH₄OH + CO₂ — > (NH₄)₂ CO₃ + H₂O

• Successive reaction with CO₂ will produce ammonium bicarbonate.

  (NH₄)₂ CO₃ +CO₂ + H₂O —- > 2NH₄HCO₃

• With further addition of Brine and carbon dioxide and by cooling the solution, sodium bicarbonate will get precipitated and ammonium chloride will be formed.

  2NH₄HCO₃ + 2NaCl — > 2NaHCO₃ + 2NH₄Cl

• The sodium bicarbonate crystals thus formed are separated from the mother liquor by filtration.
• The filtered Sodium bicarbonate crystals are then decomposed thermally into Sodium Carbonate, Water and Carbon dioxide.

  2NaHCO₃ — > Na₂CO₃ + H₂O + CO₂

The CO2 liberated is recovered in the process and recycled for use again.

To be Contd….