1.What is the concentration control range for alkaline washing of cold-rolled coils? Why is this range necessary?
The concentration of the alkali solution (as NaOH) for alkaline cleaning of cold-rolled coils is typically controlled between 3% and 5%.
This range is chosen based on the following balance considerations:
Too low a concentration (<3%): Insufficient saponification rate, incomplete grease removal, and residual oil on the strip surface will affect subsequent annealing and coating quality.
Too high a concentration (>5%): Although the saponification reaction is accelerated, excessively high NaOH concentrations reduce soap solubility, causing undissolved soap to re-adhere to the strip surface, thus reducing degreasing efficiency and increasing reagent costs and subsequent rinsing burden.
According to engineering practice, a concentration range of 3% to 5% achieves good cleaning results; further increasing the concentration does not significantly improve the cleaning effect and reduces cost-effectiveness.
2.What is the principle behind controlling the concentration of alkaline detergent? What is the relationship between concentration and cleaning effect?
The core principle of alkaline washing concentration control is based on the chemical balance between saponification and emulsification. Alkaline solutions (mainly composed of NaOH, Na₂CO₃, Na₂SiO₃, etc.) remove grease from the steel strip surface through the following mechanisms:
Saponification: The alkali reacts with animal and vegetable oils (fatty acid glycerides) to produce water-soluble glycerol and soap (sodium fatty acids), causing the grease to detach from the steel strip surface. Increasing the alkali concentration accelerates the saponification reaction rate.
Emulsification: Surfactants emulsify and disperse non-saponifiable greases such as mineral oil into the cleaning solution; appropriate concentration enhances emulsification.
3.How can the concentration of alkali solution be monitored online during production? What are some commonly used detection methods?
The production site primarily employs the conductivity method for online detection and automatic control of alkali solution concentration. The principle is that when the solution temperature and pressure remain constant, there is a one-to-one correspondence between the solution's conductivity and the ion concentration (i.e., alkalinity).
The workflow of the conductivity method is as follows:
A conductivity meter is installed on the circulation pipeline to measure the conductivity value of the alkali solution in real time.
Through a pre-established "conductivity-concentration" working curve (which needs to be calibrated in the laboratory for specific units and degreasing agent formulas), the conductivity is converted into a concentration value.
The PLC control system automatically adjusts the addition of concentrated alkali solution and pure water based on the deviation between the measured concentration and the set value, achieving closed-loop control.
The importance of temperature compensation: Since temperature directly affects conductivity, the detection system must have a temperature compensation function. Modern online detection systems typically set the temperature compensation range to 0℃~85℃, achieving a concentration control accuracy of ±2g/L.
Emerging technologies: Some advanced production lines have begun to adopt in-situ Raman online detection systems, which achieve more accurate component analysis by establishing a model of the effective components of alkali solution, with a detection accuracy rate of over 90%.
4.What quality defects can result from improper control of alkali concentration?
When the concentration is too low:
Incomplete degreasing results in residual oil and iron powder on the strip surface. During subsequent annealing, the oil will carbonize, forming "oil spots" or surface discoloration.
This affects coating adhesion and may lead to incomplete galvanizing during hot-dip galvanizing.
When the concentration is too high:
Soap solubility decreases, and calcium soap precipitates adhere to the strip surface. After being squeezed by the squeeze rollers, these precipitates form "alkali washing black spots"-irregularly distributed fine lines containing high levels of Ca and P.
For steels containing alloying elements such as silicon and manganese, excessively high concentrations may exacerbate selective corrosion.
This increases alkali consumption and the burden on subsequent rinsing, raising wastewater treatment costs.
5.How can precise automatic control of alkali concentration be achieved in production?
Automatic Alkali Dispensing Mechanism:
A concentration sensor is installed in the alkali dispensing tank. When the concentration is detected to be below the set lower limit, the control system automatically opens the dispensing hopper valve to add concentrated alkali solution, while simultaneously replenishing pure water until the concentration returns to the set range.
Dispensing trigger conditions include: conductivity dropping to a set value (due to alkalinity consumption), or the circulating tank level dropping to a set value (due to periodic discharge of dirty liquid).
Level and Concentration Linkage Control:
Six control points are set for the circulating tank level (LLL, LL, L, H, HH, HHH). The difference between the H and L levels directly affects concentration stability-a large difference leads to excessive concentration fluctuations, while a small difference results in frequent water and liquid replenishment.
Reverse Flow Process Optimization:
The advanced production line adopts a design where the cleaning agent flows in the opposite direction to the strip steel movement. New alkali solution is prepared in the electrolytic cleaning section and then flows sequentially to the spray washing section, allowing the alkali solution to be utilized and gradually become dirty, ensuring cleaner water in the subsequent tanks and improving cleaning quality.