1.What is the relationship between hydrochloric acid concentration and temperature?
Hydrochloric acid concentration: Typically controlled between 15% and 25% (weight percentage, referring to the concentration of new or added acid). The most commonly used and efficient range is 18% - 22%.
Acid solution temperature: Typically controlled between 70°C and 95°C.
Relationship: Within a certain range, increasing the temperature is more effective in promoting pickling speed than simply increasing the concentration. However, excessively high temperatures (e.g., >95°C) will lead to accelerated hydrochloric acid volatilization, resulting in significant waste, a harsh environment, and accelerated equipment corrosion.
2.What are the effects of dynamic control parameters?
Free acid concentration: This refers to the concentration of effective HCl in the pickling bath that has not yet been consumed. This is a direct indicator of pickling capacity and needs to be maintained at a certain level (e.g., 3% - 10%) to ensure sufficient pickling power.
Fe²⁺ concentration: This refers to the content of dissolved ferrous chloride in the pickling bath. It increases continuously with production.
Important impact: Lower Fe²⁺ concentration is not always better. A certain level of Fe²⁺ (e.g., 100 - 140 g/L) can increase acid density, reduce hydrochloric acid volatilization, and provide some corrosion inhibition for the substrate. However, excessively high concentrations (e.g., >180-200 g/L) will severely reduce the pickling rate, necessitating the discharge of some waste acid and the replenishment of fresh acid.
3.What are some other key factors that influence concentration settings?
Steel Grade and Hot-Rolled Coil Condition:
Ordinary Carbon Steel (CQ): Standard process can be used.
High-Strength Steel or Si-Containing Steel: Its iron oxide scale (especially Fe₂SiO₄) is more difficult to dissolve, potentially requiring higher acid concentrations and temperatures, or longer pickling times.
Oxide Scale Thickness and Structure: Different hot rolling temperatures and coiling temperatures result in different oxide scale thicknesses and densities (e.g., blue brittle oxide scale is difficult to pickle), requiring process adjustments.
Production Line Speed:
The faster the speed, the shorter the immersion time of the strip in the acid bath, requiring higher acid concentrations and temperatures to ensure oxide scale removal within the specified time.
Pickling Mode:
Shallow Tank Turbulent Pickling: Currently the mainstream method, using powerful acid spray to improve efficiency, allowing for relatively lower concentrations and temperatures.
4.What is the purpose of controlling the concentration of hydrochloric acid during pickling?
Completely remove iron oxide scale while minimizing the risk of over-acid pickling and hydrogen embrittlement of the base metal, and control costs (acid consumption, energy consumption).
5.What are the criteria for adjusting the concentration of hydrochloric acid used for pickling?
Based on the steel type, line speed, and oxide scale condition, an economical, efficient, and stable combination point is found within the broad framework of HCl concentration of 15%-25% and temperature of 70-95°C.