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9.5 Industrial Chemistry: 6. Solvay Process

Extract from  Chemistry Stage 6 Syllabus (Amended October 2002) © Board of Studies, NSW.
[Edit 25 Jun 08]

Background: The Solvay process is a method of making sodium carbonate from the raw materials sodium chloride, ammonia and calcium carbonate (limestone). This process was first used by Ernest Solvay in Belgium in the 1860s in an attempt to find a use for ammonia.  Ammonia was being produced as a by-product in the coke industry. The successful manufacture of sodium carbonate also made the manufacture of soap and glass less expensive.

About 70 Solvay process plants are still in operation, however, no new plants using this method are being built as it has been replaced by heating the mineral trona (Na₂CO_3.NaHCO₃.2H₂O) where this is available and by electrochemical methods. In Australia, commercial sodium carbonate production is carried out, using the Solvay process, by a company called Penrice in Osborne, South Australia. The Pacia website has a chemical fact sheet, Sodium Carbonate , that describes the Solvay process.

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.

identify the raw materials used in the Solvay process and name the products.

• Raw materials are sodium chloride, ammonia and calcium carbonate (limestone).
  
  
• Products are sodium carbonate and calcium chloride.

describe the uses of sodium carbonate.

• Uses of sodium carbonate include:

  • manufacture of soap, glass, ceramics, paper, sodium hydroxide and sodium hydrogen carbonate (carb. soda)
  • petroleum refining
  • water softener
  • cleaner and degreaser in washing compounds
  • removing sulfur dioxide from waste gases in power stations.

  

  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.

  For any chemical step investigated in the Solvay process, identify any risk factors. You might like to list these in a table such as the one given below. Identify any potential hazards and suggest ways of dealing with them.

  To get you started,

  • In step1, heating calcium carbonate, you could look up data tables to find the decomposition temperature of calcium carbonate. Can you achieve this temperature safely with your available equipment?
  • Check the Chemical Safety in Schools package to see if all the chemicals involved are safe to use in schools. Are there any precautions listed for any of them? Are goggles required?
  • How will you produce the carbon dioxide to bubble through the ammoniated brine in step 3?
  • Are there any problems associated with the production of gases?
  • Can you dispose of any waste safely?
  • What equipment will you need for this investigation?
  • What, if any, difficulties are associated with carrying out these reactions in the school laboratory?

  

  

  identify,  given a flow chart, the sequence of steps used in the Solvay process and describe  the chemistry involved in:

  • brine purification
  • hydrogen carbonate formation
  • formation of sodium carbonate
  • ammonia recovery.

  Brine purification

  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)   +    CO₃^2–(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)

  • Summarise by drawing up a flow chart to help you understand the steps involved in this process.
    
    On the flow chart, use a key to show:
    • raw materials
    • products sold
    • waste products.

  

  process information to solve problems and quantitatively analyse the relative quantities of reactants and products in each step of the process.

  1. A company in South Australia, called Penrice Soda Products Pty Ltd, produces 325 000 tonnes per year of soda ash (sodium carbonate). How many tonnes of calcium carbonate are needed to produce this?  Take the overall equation as,

     CaCO₃(s)   +   2NaCl (aq)       Na₂CO₃(aq)   +   CaCl₂(aq),

     Answers

  2. Evaporative basins at Dry Creek near Adelaide produce an average of 650 000 tonnes per year of salt.  This is purified, then dissolved to form a saturated brine solution that is pumped to the Solvay plant.

     Ammonia is dissolved in the brine solution and then the ammoniated brine is reacted with carbon dioxide.

     1. Write an equation for this reaction.
     2. If 50% of the original salt is sodium chloride, what mass of ammonia will be needed to react with it?

     Answers

  

  use available evidence to determine the criteria used to locate a chemical industry using the Solvay process as an example.

  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.

  Sodium carbonate in Australia. Chemlink

  

  discuss environmental issues associated with the Solvay process and explain  how these issues are addressed.

  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.

  

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