Descaling Chemicals : How To Remove Silicate Scales And How To Make Alkaline Cleaners Solutions
Introduction To Descaling Chemicals
Formulations based on Ehylenediamine Tetraacetic Acid form soluble chelates of iron in ammoniacal solutions at pH of 8-10,9. They are widely used for mill scale removal in new equipment as well as in the removal of deposits of magnetite formed in high pressure boilers in operation.
Ammonium citrate solutions rapidly dissolve iron oxide at pH of 3.5 to 4. If excess citrate ion is present the pH can be raised to about 9 without precipilation of the iron. At pH of 9, copper can be dissolved in the solution by adding an oxidant such as sodium nitrite. These chemicals are widely used in descaling of high pressure boilers where the deposits are primarily magnetite, Fe304 and copper.
For the removal of copper in a matrix of iron oxide in high pressure boilers, copper complexing agents are used which are highly functional in hydrochloric acid solutions.
By complexation of the copper in the acid solution, the undesirable electroplating of metallic copper on the freshly cleaned steel surfaces of the boiler tubes and drum is prevented.
Emulsion technology has also been applied to solvents for descaling in refineries and petrochemical plants. An inorganic portion of the scale may be admixed with a highly organic oily material. An example would be the removal of an acid soluble scale such as ferrous sulfide embedded in an oil matrix.
The emulsion would contain an organic solvent such as kerosene emulsified with a dilute solution of hydrochloric acid. The emulsification is accomplished by using a proper surfactant. The combined dissolving action and-detergency make such emulsions a time saving and useful means of descaling under these conditions.
For removing silicate scales a two-stage treatment is required.
In the first stage the scale is treated with dilute caustic solution for extended time at 200 to 240°F. This treatment softens the scale.
In the second stage the scale is removed with a strong mineral acid such as hydrochloric acid.
A small amount of ammonium bifluoride is added to the acid.
The ammonium biflouride is converted into hydrofluoric acid when added into hydrochloric acid. The softened silicates are rapidly decomposed by the fluoride ions.
The decomposition reaction is as follows:
Si02 + 6HF -.> H2SiF4 +2H20
The preliminary alkaline treatment step is not essential in all the cases.
It all depends upon the thickness and composition of the silicate scale.
Alkaline Cleaning Solutions
Metals are cleaned in strongly alkaline solutions to remove grease, oil and foreign matter, rust and corrosion, scale of calcium carbonate and other earth substances. Such treatment is necessary before painting, enamelling, electroplating etc.
The alkaline cleaner is usually in the paste or powder form.
The principal methods of alkaline treatment are soaking. spraying and electrolytic or any combination 4 them.
The basic ingredients of alkaline cleaners may be divided into five groups:
1.Basic alkalies;
2.Phosphates;
3.Silicates;
4. Synthetic organic surface active agents; and
5. Miscellaneous chemicals
The basic alkalies are caustic soda, soda ash and sodium bicarbonate.
The various phosphates are orthophosphate, pyrosphate, polyphosphate and meta- phosphate.
The various silicates are orthosilicate, sesquisilicate. metasilicate and liquid silicates also known as water glass.
The synthetic organic surface active agents are sulfated alcohols, organic sulfonates, polyglycol ethers etc.
The miscellaneous chemicals include borax, sodium sulphate, pumice. potassium salts, sodium salts, etc.
Some formulations of alkaline cleaners are given below:
Alkaline Cleaners Formulation I :
Trisodium phosphate =35.0
Soda ash =45.0
Sodium silicate =15.0
Sodium chromate =5.0
The trisodium phosphate and soda ash in the above formulation furnish a buffered alkaline base. The colloidal silicate provides protection against caustic attack and sodium chromate acts as an oxidation inhibitor on surfaces of the metals like aluminium and iron.
Alkaline Cleaners Formulation 2 :
Soda ash = 60.0
Sodium orthosilicate = 10.0
Disodium pyrophosphate = 20.0
Silicate of soda = 5.0
Sodium bichromate = 5.0
In this formulation sodium orthosilicate is used for its emulsifying action and to improve soil suspension; disodium pyrophosphate bullets the alkalinity and aids in removing rust and scale; silicate of soda acts as a protective colloid to prevent attack by the alkalies especially on tin and aluminium; and the bichromate acts as a corrosion inhibitor.
Various surface active agents are used in alkaline cleaners.
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