What is Soda Ash
Soda ash, or in chemical name known as sodium carbonate (Na2CO3), is an alkali chemical extracted from the mineral trona or naturally coming about sodium carbonate-bearing brines (both called as natural soda ash), the mineral nahcolite (called as natural sodium bicarbonate, from which soda ash can be generated), or produced from one of several chemical processes (referred to as synthetic soda ash).
The general classification of soda ash production processes currently in commercial use is as below.
Refining of natural soda ash
Carbonation of caustic soda
By-product from alumina production
Synthetic production using salt, carbon dioxide and ammonia as raw materials
The first method is applied in the United States and Kenya for soda ash discovered in underground deposits or in lakes. The second process generates soda ash through the reaction of caustic soda (a by-product of chlorine production) with carbon dioxide but because of uncertainty regarding the supply of caustic soda this process is not used for wide scale production. As of the Soviet Union which employed the third process, alumina is gained from bauxite and it obtained soda ash as a byproduct. It is estimated that the Soviets are producing 500,000 tonnes per year of soda ash using this method. And the fourth method is generally known as the soda ash synthesis process mostly used, which have four methods namely:
Full Ammonium Chloride (AC) Co-production Process
Partial Ammonium Chloride (AC) Co-production Process
A. SOLVAY PROCESS
This process is also known as the Ammonia Soda Process, the oldest of the three. The major raw materials are salt and limestone, which has the reaction as follows.
CaCO3 + C → CaO + CO2 (1)
CaO + H2O → Ca(OH)2 (2)
NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl (3)
2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O (4)
2NaHCO3 → Na2CO3 + CO2 + H2O (5)
and may be simplified to
2NaCl + CaCO3 → Na2CO3 + CaCl2 (6)
The major reaction in the production process is (3) when salt, ammonia and carbon dioxide react to form dense ash and intermediate ammonium chloride. The dense ash is segregated by settling. In the reaction (4), ammonium chloride is decomposed from the mother liquor using slaked lime. Recovered ammonia is recycled to (3) and stock is needed only to make up for loss. Dense ash is heated in a calciner and, as shown in (5), soda ash is formed with carbon dioxide which is recycled.
The Solvay Process has insufficiency in that energy is required to produce heat for (4), wherein the yield of salt is less than 73%, and there is no appropriate use for byproduct calcium chloride.
The decomposed mother liquor contains calcium chloride, unreacted salt, slaked lime and calcium carbonate. Once there is demand for calcium chloride, the above procedure will be applied for the production of purified calcium chloride. The mother liquor and carbon dioxide react to release the slaked lime as calcium carbonate, which is settled in the thickener and removed from the system. Top liquor of the thickener is assigned into the evaporator, where the crystal ammonia is separated as a product; in time of this evaporation operation salt is also recovered.
Generally, there is no sizable amount of requirement for calcium chloride, and hence the above-mentioned procedure will not be taken into consideration. Alternatively, the mother liquor is disposed of in a waste pond, where the insoluble materials are settled and top liquor is left behind.
B. HOU PROCESS
The Chinese chemist Hou Debang developed this process in the 1930s. It is similar to the Solvay process and mostly used in smaller coal-fired plants in China, which plants have a typical annual production capacity of 210,000 tonnes and it shows 25% of global production capacity. The Hou process does not yield calcium chloride as a byproduct, however rather ammonium chloride (a fertilizer).
The preceding steam reforming byproduct carbon dioxide was inflated through a saturated solution of sodium chloride and ammonia to produce sodium bicarbonate by the reaction as follows.
NH3 + CO2 + H2O → NH4HCO3 (1)
NH4HCO3 + NaCl → NH4Cl + NaHCO3 (2)
Due to the low solubility, the sodium bicarbonate was gathered as a precipitate and then heated to generate pure sodium carbonate similar to the last step of the Solvay Process.
2NaHCO3 → Na2CO3 + CO2 + H2O (3)
The Hou Process is more energy-intensive than the Solvay Process, and utilizes roughly 14.25 GJ of energy per tonne of soda ash.
C. FULL AMMONIUM CHLORIDE (AC) CO-PRODUCTION PROCESS
This process has been evolved in Japan where the salt resources are essentially limited, and has been commercially used in that country. The advantage is requiring low energy because heat for recovery of ammonia is not necessary, and the production cost of soda ash is reduced when there is demand for ammonium chloride. The reactions are as follows.
NaCl + CO2 + NH3 → NaHCO3 + NH4Cl (1)
2NaHCO3 → Na2CO3 + CO2 + H2O (2)
which can be simplified to
2NaCl + CO2 + 2NH3 + H2O → Na2CO3 + 2NH4Cl (3)
The reaction (1) is operated in two stages: carbonation and crystallization of ammonium chloride.
The mother liquor from the sodium bicarbonate separator reacts with ammonia, and it is mixed with crushed salt to gain ammonium chloride. The formed ammonium chloride is segregated by cooling crystallization followed by the thickener. Top liquor of the thickener is reprocessed back into the ammonia absorption section, then the crystal ammonium chloride is prilled and dried to be as the final product.
To decompose sodium bicarbonate to carbon dioxide and sodium carbonate (soda ash), it is heated in the calciner as shown in reaction (2). Later, carbon dioxide is recycled to the carbonation section.
In this process, the recycling solution is increased due to the appearance of moisture and impurities in the crude salt, and the impurities are accumulated; consequently it is required to blow down a certain amount of recycling solution so as to dispose of the impurities in the system.
D. PARTIAL AMMONIUM CHLORIDE (AC) CO-PRODUCTION PROCESS
This process is almost same with the Full Ammonium Chloride Co-production Process besides when the demand for ammonium chloride is limited, the part of ammonia recovery is added to Full Ammonium Chloride Process in order to regain ammonia by putting quicklime to waste chlorine as calcium chloride and recycle ammonia back to the process, as these are conducted in the Solvay Process. Compared to the Solvay Process, this process has advantages of higher salt yield and lower energy consumption.
The Uses of Soda Ash
Soap and Detergent Industry Soda ash is an important ingredient in soaps and detergents. Therefore, it functions as a builder in the formulation of the soap or detergent, to achieve the desired level of soil removal. It is also beneficial as an agglomerating aid, as it acts as the carrier for surfactants. It is also the source of alkalinity for pH adjustment.
Food Industry Soda Ash is widely used in the food industry as an acidity regulator, anti-caking agent and as a stabilizer.
Chemical Industry Light soda ash is used in the production of many chemicals. It is a source of sodium ions in the production of sodium phosphates, sodium carbonate, sodium silicates, chrome chemicals and photographic chemicals.
Other Applications Soda Ash is used as an electrolyte as it a good conductor and hence can be used in electrolysis. It is also used in small scale dyeing industries. It ensures chemical bonding of the dye to the cellulose fibres.
EMERGENCY OVERVIEW: CAUTION! MAY CAUSE EYE, SKIN AND RESPIRATORY TRACT IRRITATION. Avoid contact with eyes, skin and clothing. Avoid breathing airborne product. Keep container closed. Use with adequate ventilation. Wash thoroughly after handling. Amber white powder. Slippery when wet. A nuisance dust.
Health hazards, general: Particulates may cause mechanical irritation to the eyes, nose, throat and lungs. Particulate inhalation may lead to pulmonary fibrosis, chronic bronchitis, emphysema and bronchial asthma. Dermatitis and asthma may result from short contact periods.
Inhalation: May be irritating to the respiratory tract if inhaled.
Ingestion: May cause gastric distress, nausea and vomiting if ingested.
Skin: May be irritating to the skin.
|Loss on heating||0.80(max) %|
|Water insoluable||0.04(max) %|