Inline measurement of acid strength and iron salts in the pickling bath

Measuring density and concentration in liquids with ultrasound offers some particular advantages compared to other methods. Here are the most important aspects:

1. high precision and reliability

• Ultrasonic measurements are extremely accurate because the speed of sound in a liquid correlates directly with its density and concentration.
• The measurements are reproducible and stable, even under fluctuating process conditions.

2. real-time measurement for continuous processes

• In contrast to laboratory analyses, the measurement takes place inline and in real time. This means that processes can be adjusted immediately
• This is particularly advantageous in the chemical industry, food production or metal processing, where fast reactions to concentration fluctuations are required

3. low maintenance and durable

• No moving parts means less wear and a long service life
• Sensors are usually robust against deposits, dirt and aggressive media

4. independent of optical influences

• In contrast to optical methods, ultrasonic sensors are insensitive to turbidity, color or gas bubbles
• This makes the method particularly suitable for difficult process conditions, such as in the petrochemical industry or with heavily contaminated liquids

5. versatile in use

• Measurements are possible at high temperatures, pressures and in aggressive media
• Particularly valuable for industries such as chemicals, pharmaceuticals, oil & gas, food, metal processing and solvent recovery

6. combination with temperature measurement for more precise results

• As the speed of sound is also temperature-dependent, ultrasonic sensors are always equipped with integrated temperature compensation
• This allows even more accurate concentration measurements to be carried out

7. direct concentration determination for complex liquids

• In addition to pure density measurement, ultrasonic technology can determine specific concentrations of multi-component systems, e.g. mixtures of water, alcohol, acids or oils
• Particularly useful in chemical production, monitoring of alkaline gas scrubbers and metalworking (pickling solution, electroplating, cleaning agents)

Conclusion

Ultrasonic measurement is a precise, reliable and low-maintenance technology that is ideal for industrial process monitoring. Its robustness, real-time capability and versatility make it superior to many other measurement methods.

A pickling bath is a tank or container filled with an acidic solution used to remove oxide scale, rust, and other impurities from the surface of steel and other metals. This bath is a key component of the pickling process, ensuring a clean and smooth surface before further processing such as galvanizing, coating, or painting.

Components of a Pickling Bath

1. Acid Solution

   • The type of acid used depends on the material being processed and the desired cleaning effect:

     • Hydrochloric Acid (HCl) - Fast-acting, commonly used in steel pickling

     • Sulfuric Acid (H₂SO₄) - More economical but slower, often heated for better efficiency

     • Nitric-Hydrofluoric Acid (HNO₃ + HF) - Used for stainless steel to remove oxides and passivate surfaces

     • Phosphoric Acid (H₃PO₄) - Used when a protective phosphate coating is required

2. Inhibitors (Optional)

   • Additives that reduce acid attack on the base metal while allowing the removal of scale and rust

3. Temperature Control System

   • Some pickling baths are heated to improve reaction efficiency, especially for sulfuric acid pickling

4. Rinsing and Neutralization Section (Optional)

   • After pickling, steel is often passed through a rinsing bath with water and sometimes an alkaline solution (e.g., sodium carbonate) to neutralize any remaining acid

5. Ventilation and Fume Extraction

   • Acid baths generate fumes that can be hazardous, so industrial setups often include fume scrubbers or ventilation systems

How a Pickling Bath Works

1. Steel or metal parts are immersed in the pickling bath

2. Acid reacts with oxides and dissolves them, cleaning the surface

3. The material is rinsed with water to remove residual acid

4. If needed, it is neutralized with an alkaline solution

5. The cleaned steel is dried and sent for further processing (e.g., galvanizing, painting)

Types of Pickling Baths

• Static Pickling Baths: Used for batch processing of steel parts, sheets, or coils

• Continuous Pickling Baths: Used in high-speed steel production lines where metal moves continuously through multiple acid baths

• Electrolytic Pickling Baths: Uses an electric current along with acid to enhance scale removal

Ecological Aspects

• Acid waste must be treated before disposal

• Spent acid can be regenerated (especially hydrochloric acid)

• Fume extraction systems are required for worker safety

Pickling is a chemical process used to remove impurities, such as oxides (scale), rust, and other contaminants from the surface of steel. It is commonly used in steel production and manufacturing to ensure a clean, smooth surface for further processing like galvanizing, painting, or coating.

Step-by-Step Pickling Process

1. Pre-Cleaning (Optional)

   • Before pickling, steel may go through mechanical cleaning or degreasing to remove oils, dirt, and loose scale

2. Acid Bath (Pickling)

   • The steel is immersed in a bath of acidic solution, typically:

     • Hydrochloric acid (HCl) - Fast and efficient, commonly used in modern pickling lines

     • Sulfuric acid (H₂SO₄) - Cheaper but slower and requires heating

     • Mixed or organic acid baths - Used for specialized applications with lower environmental impact

   • The acid reacts with the oxides and scale, dissolving them from the surface

3. Rinsing

   • After pickling, the steel is thoroughly rinsed with water to remove residual acid

4. Neutralization (Optional)

   • Some processes include a neutralization step using an alkaline solution (e.g., sodium carbonate or sodium hydroxide) to prevent acid carryover

5. Passivation (Optional)

   • A passivation layer (such as phosphate or chromate treatment) may be applied to prevent immediate rusting

6. Drying and Final Processing

   • The cleaned steel is dried and can undergo further processing like galvanizing, painting, or cold rolling

Types of Pickling

• Batch Pickling: Steel is processed in batches, commonly used for coils, sheets, and fabricated parts

• Continuous Pickling: Used for high-speed processing of steel strips in large production lines

• Electrolytic Pickling: Uses an electric current to enhance the removal of oxides

Summary

• The pickling process involves dipping hot rolled steel into a solution called pickle liquor to remove surface impurities and iron oxide scale

• The solution used is typically hydrochloric acid, but steels with high carbon content may require additional acids like nitric acid, sulfuric acid, or hydrofluoric acid

• The pickling process removes 1-3% of the steel's mass, making it difficult to work with and paint

• The left-over pickling liquor, called pickling sludge, is often neutralized with a base and thrown away, but can also be used to recover useful products like hydrochloric acid and ferric oxide

• The pickling process is crucial for removing iron oxide dust and other surface impurities from metal surfaces

Monitoring acid strength in a pickling bath is critical to maintaining efficiency, controlling metal dissolution rates and ensuring product quality. There are several methods for determining the strength of an acid solution:

Sonic velocity measurement (ultrasonic method)

• Principle: Measures the speed of sound in an acidic solution to determine the concentration
• Method: An ultrasonic sensor sends a sound wave through the liquid; the transit time is recorded and compared with calibration data
• Best suited for: Continuous real-time monitoring of acid concentration in industrial pickling baths
• Advantages:
  ✔️ Non-destructive and fast - No sampling required
  ✔️ Highly accurate - No interference from dissolved metal ions
  ✔️ Real-time monitoring - Suitable for automated process control
• Disadvantages:
  ❌ Requires initial calibration for each acid type and temperature (included in LiquiSonic)

pH measurement

• Principle: Measures the hydrogen ion concentration in the solution
• Method: A pH meter or indicator strips show the acidity level
• Best suited for: Quick, rough estimation of acidity
• Advantages:
  ✔️ Simple and quick - Requires minimal equipment
  ✔️ Low cost - Simple pH strips or meters are affordable
• Disadvantages:
  ❌ Not accurate for strong acids - pH value does not indicate exact acid concentration
  ❌ Influenced by temperature and impurities
  ❌ Extremely high maintenance and material costs (running costs)
  ❌ Untrustworthy readings, as often worn out after just a few days and calibration required

Measuring the specific gravity

• Principle: The concentration of an acid correlates with its density (g/cm³)
• Method: A hydrometer is used to measure the specific gravity of the solution, which is then compared with reference tables
• Best suited for: Rapid field tests of H₂SO₄ and HCl baths
• Advantages:
  ✔️ Quick and easy - no chemical reagents required
  ✔️ Non-destructive - no need to neutralize or discard samples
• Disadvantages:
  ❌ Does not distinguish between free acid and dissolved metal salts
  ❌ Temperature-dependent, requires corrections

Acid titration

• Principle: Uses a neutralization reaction with a known base (e.g. NaOH) to determine the free acid content
• Method: A measured sample is titrated with sodium hydroxide (NaOH) and an indicator to determine the acid concentration
• Best suited for: Accurate testing of acid concentration in the laboratory
• Advantages:
  ✔️ Highly precise - determines the exact concentration of free acid
  ✔️ No interference from the dissolved metal content
• Disadvantages:
  ❌ Time-consuming - Requires laboratory equipment and qualified personnel
  ❌ Sample required - Destructive test method
  ❌ No inline measured values and therefore time delay and blind runs
  ❌ Operator errors possible and employee training required

Conductivity measurement

• Principle: Measures the electrical conductivity of the solution, which increases with the acid concentration
• Method: A conductivity meter provides a measured value that correlates with the acid strength
• Best suited for: Continuous monitoring of HCl and sulphuric acid solutions (with appropriate calibration)
• Advantages:
  ✔️ Quick and easy - provides measured values in real time
  ✔️ Non-destructive - no sample loss
• Disadvantages:
  ❌ Affected by dissolved metal ions and requires frequent calibration
  ❌ Not suitable for all acids (e.g. oxidizing acids)
  ❌ No differentiation between acid and metal content

• The pickling process is essential for preparing metal surfaces for further processing, such as cold working, welding, or painting

• Pickling helps to remove waste mill scale and iron scale from hot rolled steel, improving its surface quality and reducing the risk of defects

• The pickling process is widely used in various industries, including construction, automotive, and manufacturing

• Pickling can also be used to remove surface impurities from other metals, such as stainless steel and aluminum

• The pickling process can help to improve the corrosion resistance of metal surfaces

• Monitoring the acid concentration in the pickling bath is crucial for ensuring the effectiveness of the pickling process

• The pickling bath should be cleaned and descaled regularly to prevent the buildup of iron oxide and other contaminants

• The use of automated monitoring systems can help to optimize the pickling process and reduce waste