The Solvay Process

Identify the raw materials used in the Solvay process and name the products

Before beginning this topic, it is useful to outline what exactly the Solvay process is. Defined simply, it is the major means of production of sodium carbonate (Na_2CO_3) in the modern industrial world. Sodium carbonate is also referred to as soda ash from time to time.

In the Solvay process, ammonia, calcium carbonate and sodium chloride are reacted in various stages to produce calcium chloride and sodium carbonate. This process is not brought about spontaneously, and has multiple stages of reactions in which the raw materials are introduced.

Describe the uses of sodium carbonate

As this dotpoint is virtually endless, for the purposes of any possible questions three major uses of sodium carbonate will be briefly elaborated on, with a few more uses listed.

  • Making glass- Sodium carbonate is combined with silica (as well as other ingredients specific to the intended purpose of the glass) in order to produce
  • Regulating pH- Sodium carbonate is used in order to regulate pH in many different settings. Examples include something as simple as a neutralising agent in school laboratories, as well as cooking.
  • Softening water- Sodium carbonate is used in order to soften hard water- water containing calcium and/or magnesium ions. This is particularly useful in laundry products.
  • Other applications include: Making paper, a primary standard in titrations, producing sodium hydrogen carbonate, and reducing sulfur dioxide emissions from industrial

Identify, given a flow chart, the sequence of steps used in the Solvay process and describe the chemistry involved in: Brine purification, hydrogen carbonate for- mation, formation of sodium carbonate, and ammonia recovery

Brine Purification

Brine is first obtained from a variety of sources, including underground brine deposits, sea water evaporation projects, or simply flushing underground salt deposits. Sodium carbonate is added to this solution to precipitate out any calcium and sodium hydroxide is added to precipitate out any magnesium.

The precipitates then undergo flocculation before being filtered off upon settling.

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A 30% pure solution of sodium chloride is sufficient to begin the Solvay process.

Hydrogen Carbonate Formation

Carbon dioxide gas is first bubbled through brine which is saturated with the base ammonia to produce a weak carbonic acid.

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The carbonic acid then reacts with the weak base ammonia to form ammonium and bicarbonate ions.

 

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Sodium ions then combine with bicarbonate ions to form sodium hydrogen carbonate.

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The full equation is thus:

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The sodium hydrogen carbonate produced is then filtered off.

Formation of Sodium Carbonate

Sodium hydrogen carbonate is then decomposed into its baser components of sodium carbonate, carbon dioxide gas and water vapour by introducing heat.

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The sodium carbonate is then washed and dried, and the remaining carbon dioxide recycled for re-use in the Solvay process, along with new carbon dioxide gas produced by heating calcium carbonate (limestone).

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Ammonia Recovery

The calcium oxide produced by heating limestone is then combined with water to produce calcium hydroxide.

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This solution of calcium hydroxide is combined with ammonium chloride solution and heated to recover the valuable ammonia, as well as calcium chloride and water.

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

Thermal pollution

The Solvay process is overall an exothermic process, producing considerable quantities of heat which are released into the environment. Plants near water may discharge this heat into water sources, damaging ecosystems, yet the biggest concern is inland processing plants. This concern arises as although thermal pollution may prove damaging, in general the heat is fairly well dissipated if enough water is present. In contrast, where no water is present, these plants must large quantities of heat must explore (often costly) methods of safely releasing this excess heat. Examples of such methods include the construction of pools designed specifically for the purpose of cooling ejected hot air.

Release of Ammonia

Ammonia is an air pollutant which poses a possible health threat, particularly to marine ecosystems. Well-designed monitoring systems to combat this problem are a costly, yet relatively simple solution to this problem.

Disposal of Calcium Chloride

Calcium chloride is a product which must be disposed of safely, yet cannot be done so in bulk for fear of damaging the environment. Although other countries find a use of this product for the purpose of removing the ice from streets, this use is negligible in Australia. Disposal of calcium chloride in the water sources closely connected to the ocean is an option as the sea water can effectively neutralise any residual alkalis, yet inland plants must again seek alternate means. One possibility is the burial of dried calcium chloride, yet attention must also be paid to the cost of this procedure, as well as the leaching of ions into nearby water sources.

Obtaining Calcium Carbonate and Brine

In obtaining the calcium carbonate (limestone) and brine necessary for the Solvay process, the environment can be adversely impacted by mining projects. As such, proper research and procedures (Such as an Environmental Impact Statement in some countries) should be followed to minimise long-term harm to the environment.

Disposal of Calcium Hydroxide

The calcium hydroxide remaining from the process must be safely disposed of. This is a relatively simple process, as the substance may be neutralised and disposed of relatively easily.

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

Any step in the Solvay process can be chosen for this dotpoint. One of the simplest is perhaps the decomposition of sodium hydrogen carbonate into its base components. Keep in mind that this reaction, when done in a school lab, is nothing like the industrial process which relies on much higher temperatures. However, the chemistry is still relatively similar, if not the same.

Procedure:

  1. Place a small amount of NaHCO_3 on a piece of filter paper and record its net
  2. Carefully pour the NaHCO_3 into a test tube after weighing the test tube
  3. Stopper the test tube with a rubber tube connected to the
  1. Clamp the test tube to a retort stand, and light a Bunsen burner below the test tube such the tip of the flame just touches the base of the test tube. The rubber tube should trail down into a beaker of lime
  2. Record all observations over a five minute
  1. Allow the test tube to cool, and

Expected results:

  • Water vapour should be seen to form around the mouth of the test
  • The cloudiness of the lime water indicates the presence of carbon dioxide
  • The test tube would have decreased in mass, indicating the loss of water vapour and carbon dioxide

Also note that although we chose to identify the presence of carbon dioxide gas in this experiment, realistically the carbon dioxide gas would be carefully monitored and captured for reuse in the Solvay process. Also, take the time to consider safety precautions for this experiment such as the use of protective eyewear, laboratory coats and gloves.

 

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

This dotpoint is easy to brush aside, but is important to understand as stoichiometric calculations such as the ones involved in this dotpoint will form a vital part of the HSC Chemistry course. As such, a full line of explanation of the reasoning behind stoichiometric calculations in the Solvay process is provided below.

Initially, one mole of CaCO_3 is present (100.09g). This implies:

  • One mole each of calcium oxide (56.08g) and carbon dioxide gas (44.01g) are produced from one mole of CaCO_3:

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  • One mole each of sodium hydrogen carbonate (84.01g) and ammonium chloride (53.49g) are produced from one mole of NaCl:

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  • Half a mole each of sodium carbonate (53.00g), carbon dioxide gas (22.01g) and water vapour (9.01g) are produced from one mole of NaHCO_3:

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  • One mole each of ammonia (17.03g) and water (18.02g), as well as half a mole of calcium chloride (55.49g) are produced from one mole of ammonium chloride:

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Remember- Select the appropriate equation, solve the number of moles for the appropriate substances, and calculate the mass of these substances as required.

 

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

In selecting the location for a chemical industry using the Solvay process, several criteria must be weighed up against one another and prioritised:

  • Raw materials- Brine and limestone are two raw materials which should ideally be found close to a chemical industry using the Solvay Ammonia is not as problematic as it is recycled, bringing its costs down substantially.
  • Labour- The cost and skill of the labour force in the area are relevant factors which must be given
  • Transportation costs- The proximity of the chemical industry in relation to key locations such as suppliers and buyers must be taken into
  • Disposal of waste- The site should ideally be located near a water source connected the ocean for the disposal of calcium chloride and excess

Remember- The key concerns in choosing a location for a plant using the Solvay process are raw materials, labour, transportation costs and the disposal of waste.