What is scale?

What is Scale?



What is Scale?

Scale is a coating or precipitate deposited on surfaces that are in contact with hard water.  The most common form of scale is calcium carbonate and occurs naturally as an ingredient of chalk, limestone and marble.  Water passing over and permeating through such rocks dissolves calcite and when this water subsequently flows through a water system, the calcite precipitates out in the form of a very hard scale on surfaces. 

When hard water is heated and evaporation takes place, the problems are increased tremendously.  Calcite forms ever growing layers of rock-like deposits until pipes are totally blocked.

In summary, scale is the accumulation of mineral deposits typically from a water stream.  Depending on the type of industry you are in, scale is known by many names.

Most Common Types of Scales

Calcium carbonate is by far the most common scale found in typical water processing, both in industry and in the home.  We use carbonate in many different ways in our lives.  We enjoy its beauty (pearls), we build our world with it (cement), we use it to teach students (chalk), and we live with its pains (gall stones).  And, as scale, we must treat its presence.

The most common scales (although there are others) include:

•          Calcium Carbonate (CaCO3)

•          Calcium Sulfate (CaSO4)

•          Barium Sulfate (BaSO4)

•          Silica (SiO2)

•          Iron (Fe2O3 & Fe2O4)

Stages of scale formation:

1-      Dissolution

In first stage with increasing concentration of dissolved minerals in water, phase change from stable state to unstable will occur.

2-      Supersaturation

To simplify “supersaturation”, it can best be described as scale-causing ions that barely “hang in the water”  When calcium and bicarbonate ions are hydrated, molecules are attached to the calcium and bicarbonate ions via ionic bonds, which are much stronger than the van der Waal force.  In a supersaturated solution, the calcium and bicarbonate ions are ‘partially’ hydrated by water molecules.  The harder water is, the calcium and bicarbonate ions are hydrated with much weaker hydration energy.  We conclude that in a supersaturated solution, calcium ions are barely ‘hanging in water’.

3-      Nucleation Precipitation

Primary Heterogeneous Nucleation

Primary Heterogeneous Reaction

The System contains an existing solid surface (e.g., clay, sand, pipe wall)

the scale molecules (e.g., CaCO30) adsorb to the surface

The surface lowers the energy barrier to aggregation growth continues on the new surface

Secondary Nucleation

The System already contains calcite

CaCO30molecules adsorb to calcite surface and grow off it

The calcite surface lowers the energy barrier to reaction

Lower supersaturation is required to form scale

4-      Crystal growth

Burton-Cabrera -Frank (BCF) surface diffusion model describing crystal growth

A crystal face grows via progression of equidistant steps originating at a dislocation source

Step 1–a solvated growth unit (6-coordinated) arrives at a crystal surface and adsorbs

Step 2–a) Surface migration occurs followed by a desorption back into solution, or b) a step is reached

Step 3–Incorporation into the crystal lattice at a kink site

5-      Scale