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Introduction of Iron and Steel Pickling

At present, hydrochloric acid is used as pickling medium in almost all acid-pickling production lines in iron and steel enterprises because:

Hydrochloric acid as pickling medium can dissolve iron oxide scale quickly and rarely etch the base material.With hydrochloric acid as pickling medium, the pickling time is shorter.The strip washed with hydrochloric acid has a bright appearance.Hydrochloric acid is easy to regenerate.The operation cost is low.The surface quality of acid-washed steel strip is good, it has a clean, bright, non-stained and silver-white smooth surface after acid-wash.The hydrochloric acid pickling speed is fast, and its pickling speed is about twice as fast as that of sulfuric acid pickling speed.The hydrochloric acid pickling steel strip does not need scale breaking equipment, which saves investment in equipment.There is no iron oxide scale deposit in the hydrochloric acid pickling tank and the intermediate acid tank.Hydrochloric acid pickling does not erode the body of the steel strip. It is unlikely that the steel strip will be hydrogen fragile and have few pickling defects.Hydrochloric acid pickling reduces the metal loss by 20-25% compared with sulfuric acid pickling. The high quality Fe203 powder extracted from the recovered liquid is the high-grade raw material of soft magnetic alloy, and the large particle Fe203 is used as ore.

When pickling, the hydrochloric acid pickling liquid can be adjusted according to the material properties of steel products and the requirements of metal surface pickling.The main problems in hydrochloric acid pickling are:strong corrosion;FeCl2 in waste hydrochloric acid is not well separated and has a high cost.The price of hydrochloric acid is higher than that of sulfuric acid.

When pickling, the acid solution can leach iron peroxide and interact with metal, which is caused by uneven thickness and properties of iron oxide scale.80% of the iron oxide on the surface of the plate will be removed quickly.However, it is more difficult to remove the remaining 20% iron oxide, which requires two-thirds of the time of acid washing.In this way, 80% of the bare metal will be corroded by acid for a longer time.

Effects of various acid solutions:various oxides of iron.Ferric tetroxide and ferric trioxide dissolve much faster in hydrochloric acid than in sulfuric acid solution, so the pickling speed of steel products in hydrochloric acid is much faster than in sulfuric acid.

Dissolution rate of iron metals:Iron metals dissolve more rapidly in hydrochloric acid than in sulfuric acid.However, because the acid washing speed of hydrochloric acid is much higher than that of sulfuric acid, the amount of iron metal dissolved in hydrochloric acid is much less than that in sulfuric acid within the required time to complete the acid cleaning task.For example, when steel is pickled in 10% sulfuric acid, the dissolved iron metal is about 11.5 times as much as when pickled in the same concentration of hydrochloric acid.

The effect of higher concentration of washing liquid:In hydrochloric acid, acid washing mainly relies on dissolution to remove iron oxide scale;In sulfuric acid, pickling mainly relies on mechanical peeling to remove the iron oxide scale.When pickled with sulfuric acid, about 78% of the iron oxide peels are removed by mechanical peeling.When pickled with hydrochloric acid, only 33 iron oxide scales are removed by mechanical peeling.

Effect on special steel:Different pickling methods should be used for special steel depending on different steel types. For example, if stainless steel is pickled with sulfuric acid or hydrochloric acid, the removal of iron oxide scale cannot be achieved for any longer time.Special pickling methods should be adopted for stainless steel, such as pickling with a mixed acid solution of nitric acid and hydrogenated gas acid.Alkaline washing by melting with alkali or electrolytic acid washing, etc.

In conclusion, the type of acid has a great influence on the pickling effect.Although the hydrogen generated accelerates the acid consumption, it is the dissolution of a large amount of metal that increases the consumption of acid.The uneven dissolution of metals makes the surface rough and uneven.Furthermore, hydrogen atoms may diffuse into the metal interior, resulting in a decrease in bubbles and mechanical properties (the interaction of the hydrogen-brittle metal matrix with the acid is harmless and harmful).

The concentration and temperature of acid solution directly affect the activity of acid solution.The activity of sulfuric acid is only within a certain range of concentration, and increases with the increase of concentration.However, when the concentration increases to a certain limit, the activity decreases.The control of concentration and temperature of acid solution is related to the acid washing process as follows:

H2S04 has the strongest activity at 25% mass fraction, that is, the shortest pickling time, the higher the temperature of the acid solution, the stronger the activity of the acid solution, and the shorter the pickling time.In order to improve the pickling effect significantly and save acid solution, it is not only necessary to increase the concentration of acid, but also to increase the temperature of acid solution.Especially when the acid concentration is low, increasing the temperature is beneficial to acid washing.

Generally, the acid pickling temperature of sulfuric acid is controlled between 70 80 C or higher.However, due to the heat resistance of the anti-corrosive material of the pickling tank, and in order to reduce acid gas and protect the environment, the pickling temperature should not be too high, and generally should not exceed 95 C.

When acid washing with hydrochloric acid is used, the method of increasing concentration without excessive increase in temperature should be adopted.In general, acid pickling with hydrochloric acid uses a mass fraction of 10-15%. For stainless steel, if the temperature and concentration are too high, yellow smoke of nitric oxide will come out during acid pickling. This gas is harmful to human body. Therefore, when pickling stainless steel, we should adopt an appropriate ratio and temperature, such as 10-15% HNO3 with a maximum mass fraction of 3-5% HF.Temperature is controlled between 50 and 55 degrees Celsius.

When removing gasified iron scales mechanically, a scaler is used, that is, the oxidized iron scales on the surface of the steel plate are bent repeatedly to crack and break easily.Or it can reduce the affinity between iron oxide and metal, increase the brittleness of iron oxide, which is also conducive to acid cleaning.When the iron salt content in the acid pickling solution increases, the activity of the acid solution decreases and the acid pickling time increases.If the iron salt content exceeds its solubility, the iron salt will crystallize and deposit on the surface of the product, thus greatly reducing the pickling speed.Moreover, the metal surface becomes dirty after acid cleaning.According to the relevant information, the solubility of ferrous sulfate in 89% sulfuric acid solution is 0.Therefore, when pickling with sulfuric acid, the mass concentration of iron salt should never exceed 330g/L.In actual production, the standard for acid waste stipulates that iron salt should not exceed 300g/L.

When pickled with sulfuric acid, the content of ferrous sulfate decreases, the dissolution rate of iron metals decreases, and iron products are in danger of being pickled:according to experience, when there is no ferrous sulfate at all in the acid solution, the surface of the pickled steel products will often have a black film, whereas when the mass concentration of ferrous sulfate is 10g/L, the pickled surface is normal.Therefore, when pickling important steel products, it is better not to use the new acid just prepared, but to use ferrous acid with 10% mass concentration of some other products for pickling.

Stirring acid solution is beneficial for pickling.Stirring removes the steam condensated on the surface of steel products and the oxygen bubbles attached to it, while keeping the solution circulating and the composition uniform.Stirring also keeps oxygen in the air flowing into the acid, which can be accelerated by adding human oxygen.The stirring of acid is to pass steam or air into the acid or to use a mechanically swinging pickling basket for pickling.

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