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How to Make Caustic Soda

Tom Connor
10x Curiosity
Published in
7 min readJan 23, 2022

A crucial chemical for our modern world…

Membrane cell process — Eurochlor

Summary

  • Caustic Soda, also known as sodium hydroxide, is used in numerous industrial applications including pulp & paper, organic chemical and inorganic chemical, soaps and detergents, water treatment and metal processing
  • There are three processes of manufacture, Mercury, Diaphragm and Membrane cells
  • Mercury cell — the oldest technology employed which has been largely closed down around the world being less efficient and polluting.
  • Diaphragm cell — being phased out due to use of asbestos as separator. Contains relatively high concentration of chlorides in final caustic product.
  • Membrane cell — preferred technology for new plants and now in Australia’s for its smaller plants.
  • An Electro Chemical Unit (ECU) is 1.0 tonne of chlorine plus 1.1 tonne of caustic soda, typically this is produced as 2.25 tonnes of 50% caustic soda once processed
  • The process of manufacture uses electricity and salt. In manufacture, for every 80 tonnes of caustic soda, 71 tonnes of chlorine and 2 tonnes of hydrogen (1kg = 120MJ therefore — 240GJ are co-produced; Requiring 720GJ electricity to produce).
  • Each tonne of caustic soda requires around 1.55 tonnes of salt.
  • The process has a high energy consumption, 2,200 — 2,500 kWh (9,000 MJ/t) of electricity per ECU produced (ie per tonne of sodium hydroxide). Over 40% of the end ECU cost is energy related
  • There is over 81M tonnes of caustic produced globally (imarcgroup.com), Current chloralkali market size estimated at over $50b annually (fortunebusinessinsights.com)

How the process works (ICIS)

The chlor-alkali (also called “chlorine-caustic”) industry is one of the largest electrochemical technologies in the world. It is an energy intensive process and is the second largest consumer of electricity (2400 billion kWh) among electrolytic industries. (Electrochemistry Encyclopedia)

Nearly all caustic soda is generated by the electrolysis of sodium chloride solution using one of three cell types: mercury, diaphragm and membrane cells. Mercury cells are hazardous and have almost entirely been removed from service; Diaphragm relies on and asbestos as a separator material in diaphragm cells and is being phased out. All new production is through membrane cells which now dominate the market (>80%).

The process of manufacture uses electricity and salt. In manufacture, for every 80 tonnes of caustic soda, 71 tonnes of chlorine and 2 tonnes of hydrogen are co-produced (2.25 tonnes of 50% caustic soda with each tonne of chlorine). Each tonne of caustic soda requires around 2200 kWh (9000mj/t) of electricity and 1.55 tonnes of salt.

The hydrogen is used to produce hydrochloric acid, ammonia, hydrogen peroxide, or is burned for power and/or steam production. (Wikipedia)

Only part of the caustic soda product is withdrawn from the cathode compartment. The remaining caustic is diluted to ~32% and returned to the cathode compartment. The caustic solution that remains leaves the cell at about 30% concentration before often being further concentrated to 50% away from the cell. Typically this is done with a falling film evaporator.

In the membrane process, the ion exchange membrane acts as a barrier to all gas and liquid flows, and only allows the passage of sodium ions between compartments. The sodium ions pass in hydrated form to produce sodium hydroxide in the cathode where hydrogen is given off. Chlorine gas is liberated at the anode. The membrane is a copolymer of tetrafluoroethylene or a similar fluorinated monomer. (ICIS) Only part of the caustic soda product is withdrawn from the cathode compartment. The remaining caustic is diluted to ~32% and returned to the cathode compartment.

Membrane cell process — Eurochlor
Electrochemistry Encyclopedia
Electrochemistry Encyclopedia

Market in Australia (chemlink.com.au)

Chloralkali production in Australia is primarily to produce chlorine, which is expensive and hazardous to transport — the caustic soda is a by-product. Production centres are therefore located in the plant or, in the case of titanium dioxide pigment, over the fence and used for the manufacture of titanium dioxide, herbicides, hypochlorite and water sterilisation.

Companies

Global share of Caustic Soda Application (saharabizz.com)

Global share of Caustic Soda Application (saharabizz.com)

Economic drivers

As caustic is produced along with chlorine in roughly equal volumes the economics of the respective industries that use the raw material drive the market. Typically they are counter cyclical.

Caustic soda is utilized as a raw material in a number of industries including chemical, automotive, water treatment, and food and beverage. On account of its easy availability and affordability over its substitutes, it is regarded as one of the preferred chemical compounds employed to control acidity and remove heavy metals from water. Caustic soda is also used in the production of paper wherein it helps in dissolving unwanted compounds present in the wood pulp. Similarly, it aids in separating the ink from paper fibers during the process of paper recycling.

Apart from this, it is used in the extraction of alumina, one of the most common oxides of aluminum, from naturally occurring minerals. As aluminum finds vast applications in the production of lightweight vehicles, the growing demand for the metal is spurring the growth of global caustic soda market. (prnewswire.com)

The growing production of vinyl and polyvinyl chloride (PVC) through chlorine will bolster the healthy growth of the chlor alkali market. PVC is the world’s third-largest used polymer owing to its importance in the construction industry such as pipes, window frames, and other products. The booming construction industry in developing countries can spur opportunities for the market. Similarly, the increasing application of PVC in various end-use industries will further augment the growth of the market in the forthcoming years. For instance, in electrical applications, PVC tapes and cases are extensively used due to its unique set of properties. Moreover, the growing popularity of electric vehicles can consequently accelerate the consumption of PVC and chlorine, which, in turn, will aid the expansion of the market. Besides, the exceeding demand for chlor alkali products from end-use industries will promote market growth. (yahoo.com)

Key Chloralkali Global businesses (origin country and size of chemical plant) (fortunebusinessinsights.com)

The major membrane cell technology suppliers, include: Uhde GmbH, Asahi Chemicals, and Chlorine Engineers. (Electrochemistry Encyclopedia)

Global capacity of production Caustic Soda; 2016; Thousand Ton (saharabizz.com)

References and Further Reading

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Tom Connor
10x Curiosity

Written by Tom Connor

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