Descaling process

Use In derusting and descaling processes for metal concrete admixtures to retard set and reduce water required textile applications aluminum etching and caustic bottle washing. 

One of the most common industrial methods for the production of sodium hydroxide depends on the electrolysis of brine in a diaphragm cell. The products of the electrolysis are chlorine, hydrogen, and cell liquor, which is a solution of sodium hydroxide and sodium chloride. A large fraction of the cost of commercial sodium hydroxide results from the concentration, separation, and purification of the alkali. The sodium hydroxide required in the sea water descaling process need... 

C.3.6 Molten caustic soda (NaOH) with 1.5-2.0 % sodium hydride (NaH) 1 to 20 min 370 C For details refer to Technical Information Bulletin SP29-370, "DuPont Sodium Hydride Descaling Process Operating Instructions. ... 

Salt bath descaling is the process of removing surface oxides or scale from a workpiece by immersion of the workpiece in a molten salt bath or a hot salt solution. The workpiece is immersed in the molten salt [temperatures range from 400°C to 540°C (750-1000°F)], quenched with water, and then dipped in acid. Oxidizing, reducing, and electrolytic baths are available, and the particular type needed depends on the oxide to be removed. 

Most descaling and passivation processes for steels were developed prior to the widespread use of electrochemical techniques. As a result, a variety of visual and chemical tests are widely used for determining the surface cleanliness. Chemical tests have also been established to verify the presence of a robust oxide film on austenitic and ferritic stainlesses (8). These methods are very simple to conduct in a manufacturing environment, but they are qualitative in nature and rely strongly on the judgment of the inspector. Outside of the laboratory, electrochemical methods have not been widely used to evaluate cleanliness of carbon and alloy steels after pickling. Nevertheless, they are well suited for this purpose and have been examined in considerable detail in laboratory studies. 

The process is different to that used for shagreen. The skins must be descaled, delimed and preserved, in order to remove oils and the fishy smell. They are then tanned and dyed, before being stretched and polished. Today fish leather is becoming very fashionable, with salmon skin leading the way. 

Passivation is usually part of a chemical cleaning process using a sodium nitrite solution. (Chromates, once used extensively, are now considered to be too toxic.) Austenitic stainless steels are usually passivated in air after pickling and neutrahzation. Pickling is a chemical process to descale or clean new stainless steel. (See ASTM A380 for recommended procedures.) For heavily oxidized materials, the pickling process removes the chromium-depleted surface beneath the layer of scale. The acid solutions used contain sufficient nitric acid (a good oxidizer) so that a subsequent passivation step is unnecessary. 

The flowsheet of the process for descaling sea water and simultaneously producing a high analysis fertilizer is shown in Figure 1. Wet process phosphoric acid (or sodium phosphates obtained from reaction of wet process phosphoric acid and sodium hydroxide or soda ash) and anhydrous ammonia are added continuously to the raw sea water to precipitate the scale-forming elements as metal ammonium phosphates and other phosphates. The precipitated solids are removed from the sea water by settling and the descaled sea water is pumped to the saline water conversion plant. 

Figure 1. Process flowsheet for descaling sea water and producing fertilizer...

The descaling method would have particular value in desalination processes involving distillation. Some advantage in electrodialysis is also anticipated. Scale formation is not a problem in processes involving hydrate formation and freezing. 

Propulsion, torque production, surface cleaning, and descaling Pumping fog oil and insecticide, fan/blow replacement, and flue gas recirculation Potable water heating, chemical processing, petroleum processing, and deep-fat fryers Space heating in vehicles, homes, and industry... 

It is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale from ferrous metals, copper and aluminum alloys. A solution called tarnision liquor, which contains strong acids, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes. 

---