Scale or precipitation is formed in a water system when an insoluble mineral crystallizes on surfaces of water systems such as boilers, cooling towers, and desalination plants. which leads
to scaling on the surface of the metal, and equipment damage. The main problem related to scaling in water treatment are listed below:
- Forming an insulation film leads to a reduction in heat transfer efficiency.
- Blockage in pipelines and tubes, causing a reduction in water flow and an increase in pressure drop.
- The scaling layer formation on the metal surface will be in good condition for under despite the corrosion problem.
- An increase in power consumption due to the reduction in flow and heat transfer.
- Adsorption and waste of water treatment chemicals.
Scale can be either crystalline or amorphous in nature. Amorphous can be described as a lack of an ordered structure.
Figure 1: Scale Structure Types
It is important to understand the factors involved in scale formation and the conditions affecting the solubility of a mineral, that will help in understanding the proper way to control the deposition process. Scale conditions include:
- The saturation level of the ionic species of a mineral.
- The presence of nucleation sites, such as rough metal surfaces or small particles of scale.
- Having enough contact time that enables the scale crystals to grow.
- The scale formation rate should be greater than the dissolution rate.
- The scale should have a Strength that is able to withstand the shear forces of water flow.
Scale formation takes place once mineral ions concentration reaches above the solubility of that mineral. it can be formed anywhere in the system in combination with corrosion, fouling by suspended material, or with biofouling.
Types of Scale in Water Systems:
Scale formation could be composed of a single mineral or a combination of various elements. Some organic/inorganic compounds
The most common scales found in water systems are as follows:
- Calcium carbonate.
- Calcium and zinc phosphates.
- Silica and magnesium silicates.
- Calcium sulfates.
However other scales can be formed based on the source of water used, and the operational conditions of the treated system.
Factors Affecting Scale:
Scale is normally deposited and adheres on heat transfer surfaces where the temperature is higher than other parts of system because their solubilities generally tend to decrease as the pH and water temperature increase.
The main factors affecting the scale formation are:
- Exposure time.
- Salt concentration.
- Reaction equilibria.
In diluted and unsaturated solutions, the solutes can be found as simple ions, complex ions, or molecules. When the solution reaches the supersaturated level, some of these ions or molecules can be combined to form tiny crystals that are known as crystal nuclei.
This nucleus needs to have a particle size greater than a certain size which is called a critical size to have the ability to grow and form a crystal, otherwise, this nucleus will dissolve again.
Enough induction period is required to form the crystal in the supersaturation solution.
After that, the crystal nucleus will transport from the bulk solution to a rough surface to start the crystal growth, in this stage, the growth rate will depend on the deposition rate of the solute on the surface due to electromagnetic forces.
Deposited crystals in the later stage called the aging stage, tend to aggregate together and form larger crystals that can precipitate creating the scale.
Scale inhibitors are chemical compounds that are dosed in a low concentration in the water system to prevent or reduce scale formation.
They typically contain many active functional groups of similar or different types that have the capability to bind strongly or weakly with the cations of scales or with the forming nuclei or with the growing crystal, thereby holding them in an aqueous solution. Inorganic- or organic-compound-based scale inhibitors can be used to prevent or retard the formation of scale in the water circuit.
Mechanism of Scale Inhibition:
Scale inhibition mechanism is classified into three main types:
- Prevention of crystal nuclei formation or critical nuclei formation.
- Prevention of crystal growth.
- Dispersion of crystals (prevention of crystal aggregation)
These three techniques are explained below:
- Threshold inhibition is an inhibition mechanism that works on delaying crystal growth. In this method, the scale inhibitor functions as a nucleation center that covers the growing crystals. As a result, active crystal growth sites are blocked, preventing future crystal formation.
- Crystal modification is achieved by the modification of the crystal’s surface, causing them to distort or deformed as they grow. This distortion and disorder of the crystal can minimize or stop the growth of the typically highly ordered crystals.
- The dispersion mechanism needs highly charged synthetic polymers called dispersants, the function of a dispersant is to prevent small crystals from agglomerating into larger by charge reinforcement or steric stabilization. The increase in the negative electrical charge that all particles in the solution have, leads to electrostatic repulsion and increases the stabilization of the crystals, resulting in keeping the crystals small enough to be dispersed.
Types of Inhibitors:
Scale inhibitors can be classified based on the mechanism of the inhibitor or the nature of the chemical compound as organic and inorganic.
Inorganic scale inhibitors such as phosphate compounds (polyphosphate), and organic scale inhibitors such as poly acrylic acid (PAA) and phosphonates.
Selection of the scale inhibitor is a critical step in building a treatment program as the selected inhibitor should be fulfilling the following criteria:
- Suitability to the type of scale and its saturation index.
- Compatibility with other used chemicals.
- Compatibility with operational conditions such as temperature.
- Complying with the discharge water regulations.
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