Scaling in Salt Solution Evaporation Crystallizers: Causes and Effective Descaling Methods
- Gu Zhouying
- May 6
- 4 min read
Article Contents:
Introduction
The push toward zero wastewater discharge in the coal chemical industry has brought evaporation crystallization technology into the spotlight. These systems are essential for treating high-salinity wastewater, but they come with a serious headache: scaling. Imagine trying to boil water in a kettle that keeps crusting up with minerals—that’s exactly what happens inside these industrial machines. Scaling clogs up equipment, reduces heat transfer efficiency, and can even grind operations to a halt. So, let’s dive into what causes this problem and how to fight it head-on.
Understanding Scaling in Salt Solution Evaporators
What is Scaling?
Scaling refers to the unwanted deposition of mineral solids—usually salts—on equipment surfaces. It forms stubborn, crusty layers inside evaporators, pipes, and heat exchangers, especially where heat and fluid meet.
Why Scaling is a Big Deal in Crystallizers
It’s not just an inconvenience—it’s a production killer. Scaling can:
Decrease heat transfer by over 90%
Shrink equipment volume
Causes unplanned shutdowns
Lead to costly repairs or replacements
Causes of Scaling in Evaporators
Chemical Composition Effects
The number one culprit? The ions in saltwater. When water evaporates, ions like calcium (Ca²⁺), magnesium (Mg²⁺), sulfate (SO₄²⁻), and carbonate (CO₃²⁻) hang around and eventually bond into solid scale.
Role of Calcium, Magnesium, Sulfate, and Carbonate Ions
These ions form various scale types like:
CaCO₃ (Calcium carbonate)
CaSO₄ (Calcium sulfate)
Na₂SO₄ (Sodium sulfate)
NaCl (Sodium chloride)
Example: Calcium Sulfate’s Inverse Solubility
Here’s something sneaky: CaSO₄ becomes less soluble as the temperature rises above 40°C. So, the hotter it gets, the more it wants to jump out of the solution and stick to the nearest surface.
Here’s something sneaky: CaSO₄ becomes less soluble as the temperature rises above 40°C. So, the hotter it gets, the more it wants to jump out of the solution and stick to the nearest surface.
Influence of Temperature Changes
Temperature swings can make the scale worse. As temperature increases, some salts fall out of solution fast. Sudden changes? Even worse. That’s when you get rapid, uneven crystal growth.
Operational Conditions and Their Impact
Poor operating habits (like sudden steam surges or low liquid levels) can cause boiling inside the tubes where it shouldn’t happen. This creates hotspots for scale to anchor. Constant tweaking of parameters can make the system unstable and prone to scaling.
Common Types of Scale
Calcium Carbonate (CaCO₃)
Looks chalky, forms fast, and is very common when CO₂ and hardness minerals are present.
Calcium Sulfate (CaSO₄)
Hard and crusty—it's tough to remove and forms easily at higher temperatures.
Sodium Sulfate (Na₂SO₄)
Soluble at first, but tends to precipitate at high concentration.
Sodium Chloride (NaCl)
Usually easy to dissolve, but it can form thick deposits under high load.
Descaling Methods and Suitability
Water Dissolution Method
How it works: Flush the system with hot water.
Best for: Soluble scales like NaCl or Na₂SO₄.
Pros: Simple, cheap.
Cons: Useless against hard, insoluble scales.
Acid Cleaning Method
How it works: Acid (typically HCl) reacts with carbonate scales.
Best for: CaCO₃, MgCO₃.
Pros: Very effective.
Cons: Corrosive to equipment if not handled properly.
Alkaline Boiling — Acid Cleaning
How it works: First, use a base like Na₂CO₃ to turn stubborn sulfates into carbonate forms, then clean with acid.
Best for: CaSO₄, SrSO₄.
Pros: Effective for hard-to-remove scales.
Cons: Time-consuming and more complex.
Mechanical Descaling Method
How it works: Use tools to scrape or brush off the scale.
Best for: Thick, layered scale.
Pros: Low cost.
Cons: Risk of damaging internal surfaces.
High-Pressure Water Jet Cleaning
How it works: Blast scale off with high-pressure water.
Best for: Most scale types.
Pros: Very thorough (over 99% removal).
Cons: Expensive and labor-intensive.
Advantages and Disadvantages of Each Method
Method | Pros | Cons |
Water Rinse | Simple, low cost | Limited effectiveness |
Acid Wash | Effective on the carbonate scale | Corrosive risk |
Alkaline + Acid | Great for hard scale | Complicated process |
Mechanical | Budget-friendly | Risk of surface damage |
High-Pressure Water | Excellent results | High cost, skilled labor needed |
Preventive Measures for Scaling
Optimizing Operational Parameters
Avoid temperature spikes and maintain a steady flow rate. Stable operation equals fewer surprises.
Material Selection
Use scale-resistant materials like titanium or high-alloy steels in critical areas. Smooth, polished surfaces also help discourage deposits.
Pre-treatment of Feedwater
Remove impurities before they become scale:
Filtration for particulates
Oil separators
Softening treatments
Regular Cleaning and Maintenance
Set up a schedule based on scale trends. Don’t wait for problems—clean proactively.
Conclusion
Scaling in salt solution evaporation crystallizers is no small problem—but it’s also not unbeatable. By understanding the chemistry, keeping a close eye on operations, and choosing the right descaling methods, plants can dramatically cut down on scale buildup. Combined with preventative care, these strategies lead to smoother operations, longer equipment life, and better compliance with environmental goals.
FAQs
1. What causes scaling in evaporators?
Scaling is mainly caused by salts in the water (like calcium and sulfate) that become insoluble during evaporation and stick to equipment surfaces.
2. How often should descaling be done?
It depends on water quality and system design, but most systems benefit from cleaning every 3–6 months.
3. Is acid cleaning safe for all equipment?
No. Acid can corrode sensitive materials. Always check compatibility before use.
4. What’s the best way to prevent scaling?
A mix of good feedwater treatment, stable operating conditions, and using the right materials works best.
5. Can scaling be eliminated?
Not entirely, but it can be significantly minimized with proper monitoring and maintenance.
For the right treatment system, you need the right expertise.
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