9.5.F - Sodium Carbonate


Sodium carbonate (also known as washing soda or soda ash), Na2CO3, is a sodium salt of carbonic acid. It most commonly occurs as a crystaline heptahydrate which readily effloresces to form a white powder, the monohydrate. Efflorescence, in chemistry, is the loss of water of crystallization from a hydrated salt to the atmosphere on exposure to air.

Sodium carbonate has a cooling alkaline taste, and can be extracted from the ashes of many plants. It is produced artificially in large quantities from common salt or brine. Sodium carbonate is soluble in water, but can occur naturally in arid regions, especially in the mineral deposits (evaporites) formed when seasonal lakes evaporate. It was once used to soften water before the widespread use of detergents and is now more commonly used in glass manufacture.

Sodium carbonate has three known forms of hydrates: sodium carbonate decahydrate, sodium carbonate heptahydrate and sodium carbonate monohydrate.

In this topic students:

  • Outline the uses of sodium carbonate
  • Describe the manufacture of sodium carbonate in the Solvay process
  • Discuss issues associated with the biproducts of sodium carbonate manufacture
  • Discuss factors to be considered when locating manufacturing plants


Uses

Sodium carbonate is used in the manufacture of glass, pulp and paper, detergents, and chemicals such as sodium silicates and sodium phosphates. It is also used as an alkaline agent in many chemical industries. Prior to the widespread use of detergents and their additives, it was used as a water softener during laundry. It competes with the magnesium and calcium ions in hard water and prevents them from bonding with the detergent or soap being used. Now that detergents contain builders which do the same job, sodium carboante is not nearly as popular. Called washing soda in the detergent section of supermarkets, it effectively enhances the cleaning action of soap and anionic detergents.

A summary of the main uses of sodium carbonate include:

  • Glass making - More than half the sodium carbonate is used to make glass. Bottle and window glass is made by melting a mixture of sodium carbonate, calcium carbonate and sand (silicon dioxide).

  • Water treatment - Sodium carbonate is used to soften water (precipitates out Mg2+ and Ca2+ carbonates). This is used both industrially and domestically (in some washing powders).

  • Making soaps and detergents - Often sodium carbonate is used as a cheaper alternative to sodium hydroxide.

  • Paper making - Sodium carbonate is used to make sodium hydrogen sulphite (NaHSO3) for the "sulphite" method of separating lignin from cellulose.

  • As a common base in many chemical factories because it is cheaper than NaOH.

  • Making sodium hydrogen carbonate - NaHCO3 is used in baking soda and fire extinguishers. Though NaHCO3 is produced in the Solvay process, heating it to remove the ammonia it is contaminated with decomposes some NaHCO3, so it is actually cheaper to react the finished Na2CO3 product with CO2.

  • Removing sulfur dioxide from flue gases in power stations - This is becoming more common, especially overseas where stations have to meet stringent emission controls.

 

Manufacture

In 1791, the French chemist Nicolas Leblanc patented a process for producing sodium carbonate from salt, sulfuric acid, limestone, and coal. First, sea salt (sodium chloride) was boiled in sulfuric acid to yield sodium sulfate and hydrochloric acid gas, according to the chemical equation

below

2NaCl + H2SO4 → Na2SO4 + 2HCl


Next, the sodium sulfate was blended with crushed limestone (calcium carbonate) and coal, and the mixture was burnt, producing sodium carbonate along with carbon dioxide and calcium sulfide.

Na2SO4 + CaCO3 + 2C → Na2CO3 + 2CO2 + CaS


The sodium carbonate was extracted from the ashes with water, and then collected by allowing the water to evaporate.

The hydrochloric acid produced by the Leblanc process was a major source of air pollution, and the calcium sulphide byproduct also presented waste disposal issues. However, it remained the major production method for sodium carbonate until the late 1880s.

Solvay Process



The main stages of sodium carbonate manufacture using the Solvay
process.

In 1861, the Belgian industrial chemist Ernest Solvay developed a method to convert sodium chloride to sodium carbonate using ammonia. The Solvay process centered around a large hollow tower (B) where carbon dioxide, ammonia and brine were mixed.

The carbon dioxide was supplied by a kiln where calcium carbonate (limestone) was heated to release carbon dioxide in a decomposition reaction.

CaCO3 → CaO + CO2


In A in the diagram, a concentrated solution of sodium chloride and ammonia and brine are allowed to mix in the tower. In B, as the carbon dioxide bubbled up through the ammoniated brine from the bottom, sodium bicarbonate (NaHCO3) precipitated. In neutral or basic solutions, sodium hydrogen carbonate (a.k.a. sodium bicarbonate) is less water-soluble than sodium chloride. The sodium bicarbonate precipitates according to the following chemical reaction:

NaCl + NH3 + CO2 + H2O → NaHCO 3 + NH4Cl


The ammonia is necessary to buffer the solution at basic pH. Without it, a hydrochloric acid byproduct would render the solution acidic, and in acidic solutions, the reaction cannot proceed. In industrial practice, the reaction is carried out by passing concentrated brine through two towers. In the first (A), ammonia bubbles up through the brine and is absorbed by it. In the second (B), carbon dioxide (from the kiln) bubbles up through the brine and precipitates sodium hydrogen carbonate. Crystals of sodium hydrogen carbonate form as they have a low solubility in the ammoniacal brine. This also drives the reaction to the right and increases the yield. The crystals are washed down to the bottom of the tower where it is ice cold and then vacuum-filtered. The filtrate contains a solution of ammonium chloride. This part of the process needs to be cooled as the dissolution of ammonia in water is exothermic.

The sodium bicarbonate was then converted to sodium carbonate by heating it, releasing water and carbon dioxide.

2NaHCO3 Na2CO3 + H2O + CO2


Meanwhile, the ammonia was regenerated from the ammonium chloride byproduct by treating it with the lime (calcium hydroxide) left over from carbon dioxide generation.

CaO + H 2O Ca(OH)2

Ca(OH)
2 + 2NH4Cl CaCl2 + 2NH3 + 2H2O

 

A summary of all reactions in the production of sodium carbonate by the Solvay
process and subsequent ammonia recovery.

The recovered carbon dioxide and ammonia are recycled back to the initial brine solution, which is then reused. When properly designed and operated, a Solvay plant can reclaim almost all its ammonia, and consumes only small amounts of additional ammonia to make up for losses. The only major inputs to the Solvay process are salt and limestone, and its only major byproduct is calcium chloride.

Because the Solvay process recycled its ammonia, it consumed only brine and limestone, and had calcium chloride as its only waste product. This made it substantially more economical than the Leblanc process, and it soon came to dominate world sodium carbonate production. By 1900, 90% of sodium carbonate was produced by the Solvay process, and the last Leblanc process plant closed in the early 1920s.

The overall reaction for the process is:

CaCO3 + 2NaCl CaCl2 + Na2CO3

 

Environmental Considerations

Extract from CSU HSC Online website.

The Solvay process produces less pollution than previous methods of producing sodium carbonate. The reactions take place in a tower, designed by Solvay, and by-products such as ammonia, calcium oxide and carbon dioxide are re-used. Australian supplies of sodium carbonate are manufactured at a plant in Osborne, South Australia.

Some environmental issues include:

Calcium chloride

Calcium chloride is difficult to dispose of. Some uses have been developed, e.g. as a drying agent, as an additive for concrete and to melt ice on roads, but most is waste. The discharge of calcium chloride into rivers causes an unacceptable increase in calcium and chloride ion concentrations and affects local ecosystems. At Osborne, it has been discharged into the ocean for many years.

Solid waste

In 1997, the Solvay plant at Osborne was pumping 200 tonnes per day of waste (mainly unburnt calcium carbonate, sand and clays from the kiln) into the adjacent river, forming huge sludge deposits. Although this is not toxic, it is unsightly and a nuisance as it blocks shipping channels.

Solid wastes have now been reduced considerably at Osborne and the dumping of wastes in this way ceased by 2001. The company is researching ways to use this waste such as in fertilizer, landfill and brick manufacture.

Dust control

Dust is a problem and this is being addressed by improved truck loading facilities, upgrading of dust suppression systems in the plant, keeping vehicles on the asphalt roadways, using a wetting solution to suppress dust in open areas, using bag filters to reduce dust in the bicarbonate plant and the installation of dust scrubbing systems.

Noise suppression

Noise is being reduced by enclosure of noisy areas, using silencers to dampen noise and community monitoring to identify sources of noise.

Heat

The Solvay process is exothermic, so waste water must be cooled before it is returned to rivers or ocean to prevent thermal pollution.

 

Locating Manufacturing Plants

Extract from CSU HSC Online website.

Some factors that influence the location of a chemical industry Solvay process as an example - Australia's supplies come from Osborne, SA

Proximity to supply of raw materials

Osborne is on a 35 km strip of low-lying land along the Gulf of St Vincent.

Coastal location allows easy access to sea water - pumped into ponds for purification & crystallisation.

A limestone quarry in the Barossa Valley sends a trainload of limestone to Osborne each day.

Proximity to market

Supplies the Australian region.

Availability of transport - for raw materials and finished product.

Limestone is transported by train.

48 000 tonnes of sodium bicarbonate and 325 000 tonnes of sodium carbonate transported annually throughout Australia by road, rail and sea.

Availability of housing, transport, schools & shops for workers & family

Osborne is a western suburb of Adelaide which provides these facilities.

Facilities for waste disposal

Until recently discharged into the Port River, uses are now being found e.g. for land-fill.