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Measures to prevent scaling of wastewater evaporator
Mar 03, 2020

In the process of evaporation and vaporization, the concentrated salt wastewater is easy to produce secondary steam entrainment. The saline droplets in the entrainment are attached to the screen or baffle of the defoamer, and the crystals are constantly concentrated and precipitated to form a scale layer. In serious cases, secondary steam is blocked.


With the continuous increase of the concentration of circulating concentrated salt wastewater, the calcium sulfate, calcium carbonate and silicate contained in the wastewater will precipitate in the gap of the distributor of the falling-film evaporator and form scale, resulting in the blockage of part of the distributor.


The controlling amount of crystal species in the circulating concentrated salt wastewater in the evaporator is an important monitoring index to prevent the same crystal type and low solubility salting out from fouling on the heat exchange tube (or surface) during the evaporation and crystallization process. Its control range is narrow, it is easy to fluctuate, and the monitoring and analysis results are lagging behind. Once the insufficient crystal species quantity is not found in time for adjustment, scaling phenomenon may occur.


Measures to prevent scaling


Crystal kinds: (1) through the thick salt waste water to add a certain amount of calcium sulfate and calcium chloride as a seed, use the same as the composition of the crystal surface of the affinity of composition, reduce the degree of supersaturation of calcium sulphate in the waste water, the precipitation of calcium sulfate in waste water molecules are attached to the suspension of the seed priority, rather than deposit in heating tube wall, achieved the purpose of anti-scaling.


(2) adding scale inhibitor method: by adding scale inhibitor, metal scale ions in wastewater can be chelated to prevent them from forming scale by combining with carbonate and sulfate.


(3) adding acid: add acid, adjust the PH of wastewater ≤5.0, remove carbonic acid or bicarbonate, prevent the formation of calcium carbonate scale.


(4) purification pretreatment method: the hardness and alkalinity removal process is used to remove calcium, magnesium ions, sulfate and carbonate ions in the concentrated salt wastewater to prevent the formation of calcium carbonate and calcium sulfate scale.


(5) control of solid-liquid ratio method: external heating type forced circulation evaporator in the production process, can be solid and liquid in waste water by controlling the concentration of salt than quantity in a certain range, under the action of axial flow pump, the waste water containing more crystals have a certain velocity, exchange heat pipe wall flushing action is strong, make the crystal nucleus can't form scale layer on the inner wall of the heating tube attachment.

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