Chemistry 9.5.6
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6. The Solvay process has been in use since the 1860's
1. identify the raw materials used in the Solvay process and name the products
The Solvay process is a method of making sodium carbonate (Na2CO3) from the raw materials:
- Sodium Chloride (NaCl) – Obtained from brine (salt water)
- Ammonia (NH3) – This is reused during the process so it is not really a raw material.
- Calcium Carbonate (CaCO3) – Obtained from limestone in the ground.
The reaction overall reaction is given by,
The products are sodium carbonate (Na2CO3) and calcium chloride (CaCl2). The calcium chloride is considered a waste product.
2. describe the uses of sodium carbonate
The uses of sodium carbonate include:
- Manufacture of soap
- Manufacture of glass
3. identify, given a flow chart, the sequence of steps used in the Solvay process and describe the chemistry involved in:
The overall process can be shown by the equation,
CaCO3(s) + 2NaCl (aq) Na2CO3(aq) + CaCl2(aq)
However, this reaction cannot take place in one step as calcium carbonate will not react with sodium chloride. The reaction is carried out in a number of steps. Ammonia is involved, but is recovered again so does not appear in this summary equation.
Salt water (brine) is pumped into shallow ponds, where the water is evaporated by the sun leaving salt. This is a mixture of calcium and magnesium salts as well as sodium chloride. The Ca and Mg ions must be removed.
Calcium salts are precipitated by the addition of sodium carbonate. Ca2+(aq) + CO32–(aq) CaCO3(s)
Magnesium salts are precipitated by the addition of sodium hydroxide.
Mg2+(aq) + 2OH–(aq) Mg(OH)2(s)
A flocculant is added and the precipitates are skimmed off the brine.
Production of hydrogen carbonate and sodium carbonate
Calcium carbonate is heated in a kiln to form carbon dioxide & calcium oxide
The calcium oxide is removed, to be used in ammonia recovery.
Coke is also present in the kiln, producing more carbon dioxide when heated, as well as providing heat to decompose the calcium carbonate.
C(s) + O2(g) CO2(g)
Ammonia is dissolved in the purified brine (NaCl) and carbon dioxide is dissolved in this solution. NaCl(aq) + NH3(g) + H2O(l) + CO2(g) NH4Cl(aq) + NaHCO3(aq)
Sodium and chloride ions are spectator ions, so this equation may be written as the following ionic equation:
NH3(g) + H2O(l) + CO2(g) NH4+(aq) + HCO3–(aq)
This reaction is carried out at a low temperature (0°C) so that sodium hydrogen carbonate, which is relatively insoluble at low temperatures, precipitates out. The mixture is filtered. Sodium hydrogen carbonate is washed, dried and used to make sodium carbonate. The ammonium chloride filtrate is sent to the ammonia recovery plant so that ammonia can be recovered and reused.
Sodium hydrogen carbonate is heated to about 300°C and decomposes into sodium carbonate and carbon dioxide. Sodium carbonate is removed and sold. Carbon dioxide is reused.
Ammonia Recovery
Calcium oxide (from the first step) is dissolved in water to form calcium hydroxide. CaO(s) + H2O(l) Ca(OH)2(aq)
Ammonium chloride is reacted with this calcium hydroxide forming calcium chloride and ammonia. The ammonia is reused, but calcium chloride is waste.
2NH4Cl(aq) + Ca(OH)2(s) 2NH3(g) + CaCl2(aq) + 2H2O(l)
key to show: raw materials products sold waste products.
4. discuss environmental issues associated with the Solvay process and explain how these issues are addressed
The main environmental issue associated with the Solvay process is the waste product, calcium chloride. Its discharge into rivers causes unacceptable increase in chloride ion concentration. Some calcium chloride has been used for de-icing roads, however much more waste calcium chloride is produced than can be used.
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.
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.
Uses of Sodium Carbonate
- manufacture of soap, glass, ceramics, paper, sodium hydroxide and sodium hydrogen carbonate
- petroleum refining
- water softener
- cleaner and degreaser in washing compounds
- removing sulfur dioxide from waste gases in power stations.
1. perform a first-hand investigation to assess risk factors and then carry out a chemical step involved in the Solvay process identifying any difficulties associated with the laboratory modelling of the step
2. process information to solve problems and quantitatively analyse the relative quantities of reactants and products in each step of the process
3. use available evidence to determine the criteria used to locate a chemical industry using the Solvay process as an example
Supply of Raw Materials -
Coastal location allows easy access to sea water.
Limestone/marble quarry.
Market (buyers) - Supplies whole of Australian region and access is available by road and rail.
Availability of transport (materials and product) - Sent by road, rail and sea (need to access these)
Wokers - Close to community, but far enough away from waste from the plant.
Waste - discharged into landfill, fetilisers and brick manufacture.
