The solubility products of sparingly soluble salts such calcium carbonate, calcium sulfate, barium sulfate and silica limit the recovery that can be achieved by desalination technologies such as RO. This is especially relevant in inland desalination or zero liquid discharge (ZLD) applications. The Crystalactor is an add on tool to increase the overall water recovery and thus reduce water loss and costs for concentrate treatment or disposal.

The Crystalactor technology removes calcium from water in the form of calcium carbonate pellets by dosing lime, caustic soda or soda ash as reagents. The calcium concentration can be reduced to typically 20 mg/L as Ca (0.5 mMol/L; 50 mg/L as CaCO3). The conditions in the reactor prevent removal of magnesium in the form of magnesium hydroxide. This is an important advantage in desalination applications, because no chemicals are wasted on magnesium removal.

Magnesium carbonate and sulfate are far better soluble than the equivalent calcium salts and usually do not limit the recovery that can be achieved. Magnesium hydroxide formation can easily be achieved by pH correction to a sufficiently low value. The Crystalactor removes barium by approximately the same efficiency (%) as calcium. Some silica can be removed as well by selecting the optimum pH.

The Crystalactor can be used as either a pre-treatment before RO or as a treatment of the primary RO concentrate to allow further desalination in a secondary RO.

Process

The heart of the Crystalactor treatment plant is the pellet reactor, partially filled with suitable carrier material such as sand, garnet, or small crushed pellets.

The influent is pumped in an upward direction, maintaining the pellet bed in a fluidised state. In order to crystallise calcium carbonate on the pellet bed, a driving force is created by a reagent dosage.

Usually lime, caustic soda or soda ash is applied. The pellets grow and settle at the reactor bottom. At regular intervals, a part of the largest fluidised pellets is discharged from the reactor and fresh carrier material is added. After atmospheric drying, easy-to-handle and virtually water-free pellets are obtained. It is common practice to use these pellets, for example in the steel or glass industry.