1.What are the principles and methods of alkaline spray/immersion cleaning?
Principle: This method utilizes a hot alkaline cleaning solution (typically containing sodium hydroxide, sodium silicate, sodium carbonate, surfactants, etc.) to react with the greases in the emulsion through saponification (with animal and vegetable oils) and emulsification and dispersion (with mineral oils), causing them to peel off from the steel strip surface.
Advantages: Strong cleaning power, effective on various types of greases, relatively low cost, and mature technology.
Key control points: Alkali concentration, temperature (typically 70-90℃), spray pressure, and cleaning time.
2.What are the principles and characteristics of electrolytic cleaning?
Principle: Based on alkaline cleaning, the strip steel is used as the cathode or anode, through which direct current is applied. Electrolysis of water generates a large number of hydrogen and oxygen bubbles. When these tiny bubbles are released onto the strip steel surface, they create a powerful physical scouring force, tearing away stubborn oil stains and embedded particles.
Advantages: Excellent cleaning effect, especially suitable for high-speed production lines, high surface requirements (such as automotive steel sheets, high-end appliance sheets), and difficult-to-clean rolling oils.
Key control points: Current density, electrode polarity (cathode cleaning removes oil quickly but may lead to hydrogen infiltration; anodic cleaning removes impurities well), electrolyte parameters.
3.What are the typical processes involved in a modern continuous cleaning section?
Pre-cleaning/Spraying: The surface is initially wetted and rinsed with a low-concentration hot alkaline solution to remove most of the free oil.
Alkaline Immersion and Brushing: The strip is immersed in a hot alkaline bath with brush rollers for deep chemical and mechanical cleaning.
Electrolytic Cleaning (Core Degreasing Step): The strip passes through an electrolytic cleaning tank for deep electrochemical degreasing.
Brushing: The strip is brushed again in fresh alkaline solution or a transition tank to remove contaminants loosened after electrolysis.
First-Stage Hot Water Rinse: The strip is rinsed with hot water to remove the alkaline solution.
Second-Stage Hot Water Rinse: The strip is further rinsed with even cleaner hot water.
Final Spraying/Drying: The strip is finally sprayed with high-purity hot water or deionized water, and then immediately and thoroughly dried with a hot air dryer (such as a high-pressure air knife or hot air drying oven) to prevent water stains.
4.What are the key points of management and control?
Cleaning Media Management:
Alkali Tank: Regularly monitor and replenish alkalinity and temperature. Continuously remove floating oil using a skimmer or centrifuge. Replace the tank solution periodically.
Rinse Water: Strictly control conductivity (typically <50 μS/cm or even lower). Employ a counter-current design to maintain water quality.
Process Parameter Monitoring:
Overall line speed, temperature, pressure, concentration, and current (if electrolysis is used) at each stage must be monitored and stably controlled in real time.
Cleaning Effectiveness Verification:
Online Monitoring: Rapid sampling can be performed using a surface tension tester (e.g., dyne pen/dyne solution, indirectly reflecting cleanliness).
Offline Laboratory Analysis:
Gravity Method: Wipe the strip surface with solvent, evaporate the solvent, and weigh the residual oil.
UV Fluorescence Method: If the rolling oil contains fluorescent agents, the residual oil amount can be quantitatively analyzed by detecting fluorescence intensity.
Hydrophilicity Test: Observe the spreading of water droplets on the surface.
Post-annealing observation: Simulate annealing in the laboratory and observe whether there are oil spots or carbon black on the surface.
5.What are the common problems and solutions?
Incomplete cleaning: Possible causes include insufficient alkali concentration/temperature, inadequate electrolysis current, brush roller wear, rinse water contamination, and excessive linear speed. Systematic troubleshooting is required.
Surface water stains/marks: Incomplete drying (issues with air knife angle, air pressure, and temperature) or insufficient purity of rinse water, resulting in high calcium and magnesium ion content.
Scratches: Hard particles embedded in the brush rollers or mechanical scratches on the equipment.
Environmental and cost considerations: The cleaning process generates waste alkali and oily wastewater. An "oil-water separation" and "wastewater treatment" system is essential to achieve oil recovery and ensure wastewater meets discharge standards.