In water treatment systems, gypsum scales are not as common as carbonate scales but can be catastrophic when they do occur.
Gypsum formation occurs from the simple reaction of calcium ions with sulphate ions.
Ca2+ + SO42- => CaSO4
As gypsum has a higher solubility than other scales which means that concentrations need to be reasonably high before they start to precipitate.
Causes and occurrence
Gypsum scaling is generally limited to two main environments:
- Mining operations that can have easily 4,000 up to 40,000 mg/L sulphate as well as calcium up to 1000 mg/L and 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.
- Desalination plants that have up to 400mg/L calcium and 2700 mg/L sulphate. While these are very high in regard to gypsum formation, the low typical recovery of desal plants as well as high salinity (that increases gypsum solubility) means scale formation is usually controlable
Gypsum problems can be driven by a single poor quality water, blending of two incompatible waters or by a deliberate reaction such as lime softening.
Gypsum challenges
Gypsum scale formation can be a real challenge. Firstly the amount of scale that forms can 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.
The below example shows a typical 100m3/d brackish RO plant where gypsum is moderately saturated (194%) but producing 689kg of scale which can cause failure within hours. In comparison to calcite that is supersaturated at 560% and only precipitating ~43 kg of scale per day.
Controlling Gypsum scale
Control of gypsum scale can be achieved in a number of ways but can be quite difficult. Options include pre-treatment and use of antisclants.
Lime Softening – Lime (CaCO3) or soda-ash (Na2CO3) can be added to your feed water to precipitate out calcium and reduce the saturation. This is the most common approach in treating acid mine drainage, but comes at a cost, vast amounts of reagents are required and significant amounts of precipitated solids must be removed and disposed of.
Ion exchange – The use of ion exchange resins (usually WAC resins) to again reduce calcium is also often 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 of Gypsum scales
Cleaning of gypsum scale can be a big challenge for a number of reasons. The first issue is the amount of scale that can be up to 30kg per membrane. 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 that once the gypsum precipitates, it can form a very intractable solid if its had time to consolidate. Of the common cleaners, EDTA is normally the only effective cleaning agent and it declines after even a few days.
Luckily we have a unique solution in the AWC C-238 and AWC C-239 cleaners. These are an acid and 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.
Download and read our case study on cleaning gypsum from a severly fouled RO system.
Do you have scale control problems? Contact us today to discuss how we can help solve them.
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