1、 Impurity removal: avoid equipment blockage and pollution

Suspended particles need to be thoroughly filtered: it is recommended to control the suspended solids (SS) content in the pre treated wastewater below 10mg/L, otherwise particles are prone to adhere to the heat exchange surface of the evaporator, forming dirt that affects heat transfer efficiency and even clogs equipment pipelines.

Colloidal substances cannot be ignored: If the wastewater contains colloids (such as chemical wastewater, printing and dyeing wastewater), coagulants (such as PAC, PAM) need to be added to break the colloids and prevent them from entering the evaporator with the wastewater, causing an increase in the viscosity of the concentrated solution or adhering to the surface of salt crystals, which affects the purity of the recovered salt.

Oil and fat substances must be removed: When wastewater contains oil (such as mechanical processing wastewater), it is necessary to first remove the oil through processes such as oil separation and air flotation. Oil and fat will form an oil film on the surface of the evaporator, hindering heat transfer and potentially contaminating condensed water.

2、 Hardness control: prevent scaling from affecting operation

Calcium and magnesium ions need to be treated in advance: If the hardness of high salt wastewater is high (total amount of calcium and magnesium ions>100mg/L), chemical softening (adding sodium carbonate and sodium hydroxide) or ion exchange method should be used to remove hardness. Otherwise, during the evaporation process, calcium and magnesium ions are prone to combine with carbonate and sulfate ions, forming calcium carbonate and sulfate scale, firmly adhering to the inner wall of the heat exchange tube, greatly reducing heat transfer efficiency. In severe cases, the machine should be shut down for acid washing.

Avoid reacting with other ions: When removing hardness, it is necessary to control the dosage of the agent to avoid excessive reactions between the agent (such as sodium carbonate) and other ions in the wastewater (such as barium ions), which can generate new precipitates and increase impurity content.

3、 Organic matter treatment: ensuring the quality of effluent and salt products

High concentration organic matter requires pretreatment: If the COD of wastewater is greater than 500mg/L (such as pharmaceutical and chemical high salt wastewater), advanced oxidation (such as Fenton, ozone), activated carbon adsorption and other processes need to be used to reduce the organic matter content. Otherwise, organic matter will enter the condensate with steam, causing the COD of freshwater to exceed the standard; Meanwhile, organic matter can envelop salt crystals, leading to a decrease in the purity of the recovered salt and even rendering it unusable.

Avoid high-temperature denaturation of organic matter: During the pretreatment process, high-temperature operations should be avoided. Some organic matter may denature and polymerize at high temperatures, forming difficult to remove large molecular substances that are more prone to scaling or contaminating salt products after entering the evaporator.

4、 Process adaptation: Match with evaporator type

Adjusting pre-treatment for different evaporators: If using a plate evaporator, due to the small gap between the heat exchange surfaces, the pre-treatment needs to be stricter (such as SS<5mg/L) to prevent gap blockage; If a tubular evaporator is used, its tolerance to impurities can be slightly higher, but it still needs to meet the basic pretreatment requirements.

Considering the subsequent crystallization needs: If salt (such as sodium chloride) needs to be recovered, additional control of heavy metal ions (such as lead and cadmium) content is required for pretreatment, which can be removed through chelating resin and other processes to prevent heavy metals from entering the crystallization system and causing the recovered salt to not meet industrial standards.