What is calcium carbonate?
Of all mineral scales, calcium carbonate or calcite is the most common scale impacting water treatment systems. It is common in both industrial water treatment systems and domestic homes and across the world. Calcium carbonate (CaCO3) is formed from the reaction of calcium cations and carbonate ions that are common in both natural waters and treated waters.
The formation mechanism is as follows:
Ca2+ + CO3– => CaCO3
An important part is the pH dependence of alkalinity and carbonate ions.
CO32- + H+ <=> HCO3–
HCO3– + H+ <=> H2CO3 <=> CO2(g) + H2O
Calcium carbonate is interesting in that it has inverse solubility, meaning its solubility decreases with increasing temperature. Its formation can be triggered by a change in temperature, pH, or pressure and the resulting scale can range from a thin and easily removable layer to a very hard encrustation.
The most common form of calcium carbonate is calcite, which rapidly forms a square crystal. Other forms include aragonite and vaterite, which while less common do regularly appear in water treatment systems. Aragonite is often reported to be the first phase to precipitate out of solution particularly at elevated temperatures. In terms of solubility in water, vaterite is the most soluble, and calcite the least soluble.
SEM images of (A) hexagonal vaterite, (B) dendritic aragonite and (C) rhombohedral calcite.
Calcite scale in water treatment systems
Calcium carbonate is the most common scale impacting water treatment systems and forms in:
- Tanks, pipes and drinking water distribution systems
- Cooling systems
- Hot water heaters and heat exchangers
- In mining, and oil and gas
- During desalination and RO systems
- Water and wastewater treatment
- Cation exchange resins
Build-up of calcium carbonate can reduce heat transfer, flows, and causes pressure drops across equipment. Calcium carbonate is by far the most common scale found in cooling water systems and a major operational problem during seawater desalination and solar heating systems.
Formation can be driven by a number of mechanisms including:
- Evaporation – especially in tanks at the water-air interface. Evaporation leads to scale formation
- Heating – The solubility of calcium carbonate is strongly dependant on temperature and as such is a very common scale in heat exchangers, kettles and water heater systems
- Concentration effects – Concentration of a solution, such as in a RO plant will often drive calcium carbonate formation
- pH – Because of the pH dependence of the alkalinity species, calcium carbonate scales are highly impacted by pH. The high-risk range is >8.5 where bicarbonate is converted to carbonate species
- Water blending – well waters and surface waters frequently have high alkalinity and when this blends with a water stream that contains calcium then scale formation tends to form relatively quickly
The main causes of calcite scale
In our experience while there are a number of possibilities, one of the most common causes of calcite scale are when 2 waters blend together; one with high calcium and one with high carbonate. Quite often the water will be starved of one of these ions and then a change in the mixing ratio of these waters will cause the scale to form.
If you suddenly have a problem, check your feed water.
The next key change is in the pH of the water. As mentioned, carbonate can be in a number of forms. One of the key equilibria here is between bi-carbonate and carbonate. While above pH 11 the carbonate content is at a maximum, one of the more dangerous points is pH 9-11 as this is when a small change in pH is rapidly increasing the amount of carbonate and thus the scaling tendency.
How do you identify and solve a calcium carbonate scale problem?
The good news is that calcium carbonate scales are relatively well understood and controlled. Scale inhibitors exist to control scale formation over a wide range of conditions. Additionally, acid addition is effective in controlling formation in membrane and cooling tower systems, and acid Clean in Place (CIP) will effectively remove the scales.
It is relatively easy to identify a calcite scale by looking at both the water chemistry of your feed water and the effectiveness of an acid-based CIP. If a major calcite scale problem exists, it is important to monitor the pH of your CIP solution to ensure it does not get exhausted during cleaning.