Gypsum Scale

Gypsum (or Calcium sulphate) is a hard and tenacious scale that forms as large volumes of needle like scales. While gypsum scale is not as common as calcite, they do form in particular waters and can be catastrophic when they do occur.

Gypsum Scale Properties and Occurrence

Chemically gypsum forms from the reaction of calcium ions with sulphate ions. Ca²⁺ + SO₄^2- => CaSO₄ One key point is that gypsum has a higher solubility than other scales, which means that ionic concentrations need to be reasonably high before they start to precipitate and this in turn causes a much larger volume of scale to precipitate. For this reason gypsum scales usually only occur in waters with high sulphate levels such as:

• Sea Water Desalination plants that have up to 400mg/L calcium and 2700 mg/L sulphate. While these are high for gypsum saturation, the low typical recovery of desal plants as well as high salinity (that increases gypsum solubility) means it’s usually under control
• Mining Operations that can have easily 4000 up to 40,000 mg/L sulphate as well as calcium up to 1000 mg/L frequently have issues. Acid mine drainage where sulphides in rocks and coal oxidize to form sulphuric acid are a common cause, as are mining operations that use sulphuric acid for leaching and mineral processing.

As mentioned, one of the key features of gypsum is its higher solubility that results in a large amounts of scale when it does start to precipitate. To demonstrate this we have compared the Saturation Index (the driving force for scale formation), with the Precipitation Potential (the expected mass of scale) for both calcite and gypsum in a typical small RO plant. This example is for a 100 m³/d brackish RO plant treating water with moderate hardness, alkalinity, and a reasonable sulfate level. Under these conditions, calcite is highly supersaturated at 560% of its saturation and generates around 43 kg of scale per day. In contrast, gypsum, though only moderately saturated at 194%, produces 16 times more scale—approximately 689 kg per day. Even if a small fraction of this scale adheres to the membrane, it can lead to system failure within hours.

Problems caused by gypsum

Gypsum scale formation can be a real challenge because the large volume of scale that forms will quickly block up pipes and equipment to prevent cleaning chemicals being able to be added. Secondly, the sharp needles that are typically found with gypsum can pierce and damage membranes.

Controlling calcium sulphate

There are several ways of controlling gypsum scale formation but it can be difficult due to the high concentrations of the species involved. Options include:

• Lime Softening – Lime (CaCO₃) or soda-ash (Na₂CO₃) can be added to your feed water to precipitate out both calcium and sulphate ions and reduce the saturation. This is the most common approach in treating acid mine drainage, but comes at a cost due to the large amounts of reagents and precipitated solids must be disposed of.
• Ion exchange – The use of ion exchange resins (usually WAC resins) to reduce calcium is also used to limit the saturation of gypsum. However, this is generally only economical in situations where there are no alternatives.
• Antiscalants ¬ There are some excellent gypsum antiscalants available including AWC A-108 and AWC A-110. These work by controlling the formation of gypsum crystals and making sure they don’t precipitate until after the RO plant.

Cleaning

Cleaning of gypsum scale can be a big challenge for a number of reasons. The first issue is the amount of scale that forms. It’s quite common to have membrane modules that have 30kg or more of scale in them. This blocks the brine spacers and flow paths and makes it difficult to get cleaners to where they need to be. The second issue is the resistance of gypsum to chemical cleaners. Chelating agents such as EDTA are normally the only cleaning agents that have any effect, and they are often ineffective unless applied very soon after the scaling event. Luckily we have a unique solution; the AWC C-238 and AWC C-239 cleaners. These are an acid and a base cleaner that can be used independently or in combination for a greater effect. They breakdown part of the gypsum and soften it into a slurry. This slurry then allows more penetration of chemicals and can then be removed from the system through high-speed flushing. There is a great AWC case study on this if you’re interested learning more about this.

Conclusion

Gypsum scale is a common scale that occurs in waters with high sulphate often in mining operations and can cause catastrophic failure of a membrane or WTP system. Luckily it can usually be controlled using high quality antiscalants and cleaning chemicals. If you have a gypsum scaling problem, contact Membrane Works today to find out more on how we can help improve your operation.