Caustic Soda & Applications 

Polycarbonates are transparent thermoplastic polymers that are mainly used as moulding compounds.  CD-roms and baby bottles are well known examples of this use.

The great commercial success of polycarbonate (Lexan®, Makrolon®) is due to its unique combination of properties: extreme toughness, outstanding transparency, excellent compatibility with several polymers, and high heat distortion resistance.

The production of polycarbonate can be split into two stages:

1. production of phosgene (COCl2) starting with chlorine (Cl2) and carbon monoxide (CO) according to the following reaction: CO + Cl2  à COCl2
2. reaction between phosgene and bisphenol-A in a solution of methylene chloride: Cl-CO-Cl + HO-C6H4-C(CH3)2-C6H4-OH   à  HO-C6H4-C(CH3)2-C6H4-O-CO-]80 till 100 + 2 HCl

Large quantities of caustic soda lye are added during the second stage in order to form bisphenolate and to catalyze this reaction by neutralizing the hydrochloric acid formed.
The polycarbonate obtained in this way is precipitated as a solid substance after purification and centrifuging and subsequently dried until a powder is obtained.

A cross-linked form of the sodium salt of polyacrylic acid is used as a super absorbent material in diapers and other personal hygiene products.
SAP is manufactured by the polymerization of the acrylic monomer; the main raw materials are acrylic acid and caustic soda. From the polymerization process, the gel granule is formed and sent to the drying process and surface coating before drying and packing.

Caustic soda is used in the production of zeolites.

Zeolites are crystalline, hydrated aluminosilicates with a framework structure. They have a three-dimensional polyanionic network constructed of SiO2 and Al2O3 tetrahedra linked through oxygen atoms.  The pores contain water and cations to balance the negative charge of the framework.  These cations are mainly alkali metal or alkaline earth, and can be exchanged, giving zeolites their main feature: binding cations.

A hydrothermal reaction of sodium aluminate, sodium silicate and caustic soda in an aqueous solution at elevated temperature and under high pressure is carried out for this production process.  A strong basicity is necessary to build the crystallized form between silica and aluminum ions with oxides.  Caustic soda is used for this synthesis. Zeolites containing a high amount of silica are synthesized at high temperatures and pressures, while low-silica zeolites are usually crystallized at 70-100 °C and with a pH in the range of 10-14, in a caustic soda solution. 

Different ratios between silica and aluminum contents give different types of zeolites.
Due to its excellent cation binding capacity, zeolites are used as a component in detergents; as adsorbents, for separating and purifying substances; and as catalysts, in a great number of important processes in the chemical and petrochemical industries. They are used also in a great number of applications where the cations’ binding capacity is needed.

Caustic soda is used industrially to produce sodium phosphate salts.  The different forms of these salts as monosodium phosphate, disodium phosphate and sodium tripolyphosphate are obtained by neutralization of phosphoric acid with soda ash or caustic soda.

All salts can be produced with either caustic soda or soda ash except trisodium salt. Because the basicity of soda ash is insufficient, the trisodium phosphate form must be obtained with caustic soda.

Sodium sulfite and sodium metabisulfite are industrially produced by neutralizing sulfur dioxide solution with either caustic soda or soda ash. Sodium sulfite salt is widely used in   photographic chemicals, water treatment, pulp & paper, leather and textiles.

The reaction of epichlorhydrin to produce chlorhydrin is a necessary step in obtaining epoxy resins.  This reaction needs to be catalyzed by caustic soda.  After this reaction, a further step is needed to obtain the final product.

Epoxy resins are chemical resistant coatings that are used in many applications where a chemical surface protection is needed.

Sodium aluminate is an important commercial inorganic chemical. It works as an effective source of aluminum hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as NaAlO2 , Na2O • Al2O3 , or Na2Al2O4.

Sodium aluminate is manufactured by the dissolution of aluminum hydroxides in a caustic soda solution.  Aluminum trihydroxide (gibbsite) can be dissolved in 20-25 % aqueous NaOH solution at a temperature near the boiling point.  The use of more concentrated NaOH solutions leads to a semi-solid product.  The process must be carried out in steam-heated vessels of nickel or steel, and the aluminum hydroxide should be boiled with approximately 50% aqueous caustic soda until a pulp forms.

The final mixture has to be poured into a tank and cooled; a solid mass containing about 70% NaAlO2 is then formed.  After being crushed, this product is dehydrated in a rotary oven heated either directly or indirectly by burning hydrogen.  The resulting product contains 90% NaAlO2 and 1% water, together with 1% free NaOH.

Sodium Aluminate is used for different purposes: for water treatment it is used as an adjunct to water softening systems, as a coagulant aid to improve flocculation, and for removing dissolved silica.  In construction technology, sodium aluminate is employed to accelerate the solidification of concrete, mainly when working during frost.  It is also used in the paper industry, for refractory bricks production, alumina production, etc.

Monosodium Glutamate is a fine white crystalline product,  widely used in  snack foods, canned foods and daily cooking to enhance flavor. The product starts with the fermentation of a natural material like sugar or tapioca and is then hydrolyzed by hydrochloric acid and neutralized by caustic soda.

From the native tapioca starch, it is first  digested by caustic soda at  a 2% concentration. It then reacts with acetic anhydride using HCl to adjust pH to be around 6 - 6.5. The final product is sieved, washed and dyed before packing.

The type of resin to be used should be defined by the resin producer depending on the requirements. These ion-exchange resins need to be regenerated periodically depending on water and resin characteristics.

The products regenerating these resins are caustic soda for the anion exchange resin and hydrochloric acid for the cation exchange resin.

Regeneration processes must be defined by the resin producer, but, in general, three steps for the anion resin can be mentioned:
1. Backwash rinsing with water to eliminate fines and any particle coming from the water flow,
2. Injection of a 4% aqueous caustic soda solution in Mg2+ and Ca2+  free water to avoid precipitation on the resin bed,
3. Rinsing with about 3 times the resin volumes to eliminate the caustic soda in two progressive phases, a slow one to eliminate the greater part of the caustic soda and a fast one to eliminate the last residue.

Hardness in water is the result of concentrations of calcium and magnesium ions measured in mg/l as calcium carbonate (CaCO3).  Water hardness can cause scaling problems in water heaters and soap doesn’t lather well in hard water. Also, many industrial uses need soft water.
Different processes have been developed to remove hardness from water:
• Chemical precipitation,
• Ion exchange resins,
• Reverse osmosis.

Caustic soda can be used to remove both carbonate (temporary) and non-carbonate (permanent) hardness.  Calcium and magnesium hydrogen carbonates react with caustic soda to produce insoluble calcium carbonate and magnesium hydroxide.
[Ca2+ + 2HCO3-] + 2NaOH -> CaCO3¯+ 2Na+ + CO3- + H2O
[Mg2+ + 2HCO3-] + 4NaOH -> Mg(OH)2¯+ 4Na+ + 2CO3- + 2H2O
Mg2+ + 2NaOH  -> Mg(OH)2¯ + 2Na+

Caustic soda softening is more efficient at pH of 9.0 to 9.5 for calcium and at pH of 11.0 for magnesium; therefore, extra caustic soda is added in this latter case to raise the pH.  The pH must be neutralized at the end in order to obtain the desired pH range.
It is usually not desirable (or possible) to remove all the hardness by precipitation. Typically, the minimum possible calcium hardness is about 20 mg/L and the minimum magnesium hardness is about 10 mg/L, both as CaCO3.  Therefore, about 30 mg/L total hardness as CaCO3 can remain.

Mercerization is the treatment of cotton under tension with caustic soda solution at 15 – 25 °C for 25 - 40 s.
For this purpose fibers and fabrics are impregnated within a caustic soda solution.  The concentrations normally used   for woven materials are in the range of 20 - 24% NaOH (26 - 30° Bé), and for yarn, 24 - 26% NaOH (30 -32° Bé). Treatment with concentrated caustic soda destroys the spiral form of the cellulose, enhancing its ability to take up dyes.  It also gives luster to the fabric and a good dimensional stability for the woven fabrics, among other things.
For effective mercerization, complete penetration (wetting) of the cotton fibers by the concentrated caustic solution is needed. Wetting agents are needed also to reduce the surface tension of the solution.

For a great many centuries, natural materials such as wool, cotton, linen and silk were the only resources of the textile industry.  All textile products were direct consumers of   natural materials.
However, over about the last fifty years, new fibers have been appearing on the market, where they have progressively carved out a major place for themselves.  Cellulose textiles, obtained by the viscose process (rayon, spun rayon), have taken a leading share in this area.  

Alumina is obtained by treating mineral bauxite with caustic soda under moderate hydrothermal conditions.  The purpose of this treatment is to extract the Al2O3 from the mineral.  The solubility of Al2O3 in NaOH is temperature dependent; most other components of the bauxite are quite inert in the process and the silica that does dissolve subsequently forms a nearly insoluble compound.  These features permit formation of a sodium aluminate solution, physical separation of the impurities, and precipitation of pure Al(OH)3 from the cooled solution.
The process begins with the preparation of the bauxite by blending to obtain a uniform composition, and then by grinding. The bauxite is ground  while suspended in a portion of the process solution. This slurry is mixed with the balance of the heated NaOH solution, and then treated in a digester vessel at a pressure far above atmospheric.
The produced slurry is removed from the digester and the solid bauxite residues are separated from the sodium aluminate solution in two steps so that the coarse fraction is processed separately from the fine.  Both residue fractions are washed and discarded.
After several steps, alumina Al2O3 is obtained and used as raw material for aluminum production in Hall-Héroult electrolytic cells.

The process begins with the preparation of zinc liquid by roasting zinc sulfide with the acid and passing it  through the purification unit to eliminate   other ions like copper, cadmium, cobalt and nickel. After purification, zinc liquid is electrolyzed to obtain zinc plate.

 Soap-making is the oldest application of caustic soda and was also the largest market. Caustic soda is used to neutralize fatty acid in the production of soap.

Caustic soda is used in the production of various types of detergents. Even modern washing powders are manufactured with large quantities of caustic soda. Caustic soda is being used to neutralize the excess   oleum after the sulphonation reaction.