The purpose of this report is to state the industrial production of Sodium Hydroxide, and its environmental impacts.
The three different ways of producing Sodium Hydroxide are through:
Mercury Cell
Membrane Cell
Diaphragm Cell
This report will further discuss these three different ways to produce Sodium Hydroxide as well as its Impacts on our environment.
NaOH is a white crystalline solid that is also known as caustic soda, lye, or sodium hydrate. Its chemical formula is NaOH. It is commonly available in the form of pellets, sticks, or chips, and in water solutions of various concentrations. It is a strong base and easily soluble in water, alcohol, and glycerine. Sodium hydroxide is used commonly in the manufacture of other chemicals; for example, in the production of rayon and other textiles, in the production of paper, in etching aluminium, in the production of soaps and detergents.
The principal method used for the manufacture of sodium hydroxide is electrolytic dissociation of sodium chloride. During this process chlorine gas is produced as a co product. Another method used for its manufacture is a soda- lime process. In this process a concentrated solution of sodium carbonate (soda) is reacted with calcium hydroxide (slaked lime). Calcium carbonate precipitates out, leaving a sodium hydroxide solution.
Modern methods of sodium hydroxide manufacture use electrolysis, the process of converting ions into elements by means of an electric current. The process can still be called an oxidation, since the negative charge on the chlorine ion is removed by causing it to come into contact with the anode of the cell.
The process of electrolysis requires:
- Electrodes: anode and cathode
- Solution (or molten salt): electrolyte
- Electric Current direct current
The electrodes and electrolyte are called a cell.
In the electrolysis of salt (passing an electric current through a saturated solution of salt (brine)) chlorine, sodium hydroxide and hydrogen are produced in the fixed chemical ratio of 1/1.13/0.028 (Orica 1999). This is called an Electrochemical Unit (ECU).
These products can be produced by a variety of different technologies although all accomplish the same overall chemical reaction, which basically splits sodium chloride (salt) into its component parts of sodium hydroxide and chlorine.
Sodium Hydroxide solutions are produced as a co-product with Chlorine electrolytically by three technologies: mercury cells, membrane cells, and diaphragm cells. Each of these processes utilizes NaCl salt as the primary raw material. The salt is electrolytically split using direct current (DC) electricity, resulting in Chlorine and an available sodium ion (Na+) that is reacted with water in the cell to make Sodium Hydroxide and by-product Hydrogen (Encarta 1999). The hydrogen by-product produced is used as a fuel source, sold to hydrogen customers, or to produce high purity (burner grade) Hydrochloric Acid.
Following is a brief discussion of the three electrochemical processes that are dominant in Sodium Hydroxide production:
In the Mercury Cell Process saturated brine travels down a steel trough approximately 15 meters long and one meter wide between a flowing film of mercury (the cathode) and titanium plates (the anodes). Direct current is applied between the anode and cathode (Encyclopedia 2000). Chlorine liberated at the anodes collects above the brine and is led off as a hot, wet and corrosive gas.
Sodium ions are discharged at the surface of the flowing mercury cathode, forming an amalgam of low concentration with the mercury, which flows out of the cell without reacting with the water or chlorine.
The mercury cell thus has two products:
(a) Hot, wet chlorine; and
(b) Sodium amalgam.
The soda cell or decomposer, is a cylindrical steel trough filled with graphite balls or graphite electrodes. The sodium amalgam is passed, along with pure water, into the decomposer, where it reacts to produce Sodium hydroxide as a controlled 50% aqueous solution and hydrogen gas, liberating fee mercury, which is recycled back to the electrolytic cell (Baker 1998). The graphite provides a surface that facilitates this reaction.
The two parts are often referred to as the brine cell, and the soda cell.
The reactions in the mercury are:
Brine Cell:
2Cl = Cl 2 + e
Na + e = Na
Na + Hg = Na/Hg
(Sodium amalgam)
Soda Cell:
2 Na/Hg + 2 H 2 O = 2 NaOH + H 2 + 2 Hg
(Caustic + (Hydrogen) + (Mercury) Soda)
Click here to view the Process Flow Diagram (Orica 1999)
Total equivalent energy, on a DC basis, needed to produced Caustic Soda (NaOH) via the mercury cell process is approximately 3600 kWh per metric ton.
This process produces approximately 71% of electrolytically produced Sodium Hydroxide in North America (Donnees 1996).
This process utilizes asbestos, or alternate substitutes to asbestos, to separate the co-products Sodium Hydroxide (Caustic Soda) and Chlorine. The production of 50% NaOH occurs primarily outside of the electrolytic cell. The diaphragm cell produces a very weak 'cell liquor,' which contains 12-14%, by weight, NaOH and roughly the same concentration NaCl salt. The 'cell liquor' is subsequently evaporated in a three or four 'effect' evaporation process to a final nominal concentration of 50% NaOH by weight (49-52% range). The excess salt is precipitated and filtered through the evaporation process for subsequent reuse/recycle. This process produces the lowest quality electrochemical NaOH solutions.
The quality considerations with respect to the diaphragm cell produced Caustic solutions include relatively high salt, chlorates, carbonates, and sulfates. Salt, as NaCl, concentrations are typically 1.0%, with maximums ranging from 1.1 to 1.3 weight %, depending on producer. Sodium Chlorates are typically 0.15 weight %, with a maximum of 0.3 weight %. Sodium Carbonates are typically 0.1 weight %, with a maximum of 0.2 weight %. Sodium Sulfates are typically 0.01 weight %, with a maximum of 0.02 weight %.(Plambeck 1995)
The diaphragm cell produced Caustic Soda (NaOH) is often referred to as Diaphragm Cell Grade. It is also called Commercial Grade, Technical Grade, and occasionally Technical Diaphragm or other similar combinations.
An additional 'grade' of Caustic Soda (NaOH) produced by the diaphragm cell process is the Purified Grade. The production of Purified Grade involves the further evaporation of the 50% Diaphragm Grade NaOH solution to reduce the salt concentration. The higher NaOH concentration forces precipitation of the salts, which are soluble in Caustic Soda (NaOH) solutions in an inverse relationship. The higher concentration solution is then re-diluted to the 50% concentration that is commercially available as Purified Grade Caustic Soda (Encarta 1999).
Common uses include process and wastewater neutralization, textiles production, soaps and detergents and aluminum production. These uses and applications generally will refer to the Caustic Soda as any of the various grades previously addressed, dependent on supplier's terminology.
Total equivalent energy, on a DC basis, needed to produce Caustic Soda via the diaphragm cell process is approximately 5000 kWh per metric ton (Donnees 1996).
This process produces approximately 13% of electrolytically produced Sodium Hydroxide in North America (Donnees 1996).
The membrane cell process utilizes a selective membrane that separates the Chlorine and Sodium ions. The membrane allows the Sodium ion to 'migrate' across the membrane while keeping the Chlorine gas and salt (brine) solution in a compartment on the other side of the membrane. The Sodium ion is reacted with purified water as in the mercury cell to produce the Caustic Soda (NaOH). The solution produced by the membrane cell process is nominally 33-35 weight %. Evaporation is utilized, as in the diaphragm process, to raise the concentration up to the nominal 50 weight % solution. The salt concentrations are not concentrated as significantly in this evaporation process due to the selective osmotic nature of the membranes as well as the reduced amount of evaporation required in this process opposed to the diaphragm evaporation (Twardowski 1988).
The Caustic Soda produced by the membrane cell process is most commonly referred to as Membrane Grade. It also has a growing acceptance as a Rayon Grade product in all areas outside of rayon fiber production. Other terminology that is used include High Purity and High Purity, Technical Grade.
Total equivalent energy, on a DC basis, needed to produce Caustic Soda via the membrane cell process is approximately 3360 kWh per metric ton (Donnees 1996).
Sodium
hydroxide is very corrosive. Sodium hydroxide should be kept away from drinking
water source because it may die or have severe scarring of tissues in humans as
well as animals.
Molten sodium hydroxide has characteristics as faster decomposition capability. Sodium hydroxide in the atmosphere decomposes to form sodium oxide, which is a hazardous gas. The presence of this gas in the atmosphere is a source of air pollution. Air pollution not only affects humans but also affects the environment adversely by destroying plants in the wild, putting additional stress on ecosystems.
The
decomposition of Sodium hydroxide by reaction with certain metals such as
aluminum, tin and zinc releases explosive hydrogen gas. Sodium hydroxide should
not be allowed to come in contact with these metals. This gas results in direct
poisoning of the environment.
Also,
when Sodium hydroxide comes in contact with acids and organic halogen compounds,
especially trichloroethylene, causes violent reactions. Sodium hydroxide even in
fairly dilute solution reacts readily with various sugars to produce carbon
monoxide. Carbon monoxide in turn adversely affects agriculture by reducing crop
yields.
During
the manufacture of sodium hydroxide, mercury
forms an essential part of the mercury cell process. Mercury is a very harmful
element and stringent steps are taken to control and minimize the mercury usage
during manufacture. Mercury entering or leaving the cell is washed with filtered
water to remove any impurities that may be present. This water can prove
detrimental to the aquatic life. Therefore this water should be passed through a
special mercury trap where the mercury could be recovered. Also mercury vapor
may be contained in flu gases. This gas has pollutes our environment. Therefore
this vapor should be drained and returned to the cells.
Written & Designed by: Pooja Bhardwaj
Date Issued: November 7th 2001.
