1、 Source pre-treatment: reduce salt load and impurity content

Reducing "easily crystallizable substances" and "interfering impurities" before wastewater enters the equipment is the basis for preventing blockages.

Salt separation pretreatment reduces high saturation salt concentration: If the wastewater contains salts with large solubility differences such as sodium chloride and sodium sulfate, nanofiltration (NF) membrane separation technology can be used to separate high solubility salts from low solubility salts (such as calcium sulfate), so that the wastewater entering the low-temperature evaporation equipment contains only a single or low mixed salt, avoiding crystal bonding caused by cross precipitation of multiple salts. For example, after nanofiltration pretreatment of high salt wastewater from coal chemical industry, calcium sulfate can be intercepted in advance, greatly reducing the risk of scaling in the evaporation system.

Removing suspended impurities and colloids: Impurities such as sediment and organic colloids in high salt wastewater can become "crystal nuclei" for salt crystallization, accelerating crystal attachment. The suspended solids content can be reduced to below 10mg/L through the "coagulation precipitation filtration" process in the pretreatment stage (such as adding polyaluminum chloride PAC and using quartz sand filters), reducing the probability of crystal nucleation and avoiding the formation of difficult to clean blockages caused by the mixing of crystals and impurities.

2、 Equipment structure and material adaptation: reducing the possibility of crystal adhesion from the design end

The structural design of the equipment directly affects the flow and deposition of salt, and targeted optimization of core components is needed:

Adopting forced circulation and high flow rate design: an evaporator with an "external circulation pump+guide plate" is selected to drive the wastewater to flow in the heating tube at a high flow rate of 2-3m/s through the circulation pump, using fluid flushing force to reduce the residence time of salt on the tube wall and avoid crystal adhesion. For example, the flow rate inside the heating tube of a forced circulation low-temperature evaporator is 3-5 times that of a natural circulation device, and the crystallization blockage rate can be reduced by more than 60%.

Optimize evaporator type and inner wall treatment: Prioritize the use of rising film and falling film evaporators, which rely on the film like flow formed by steam rising or liquid falling to ensure uniform contact and short residence time between wastewater and heating surfaces, reducing excessive local concentration. At the same time, the inner wall of the evaporator and heating tube are treated with "anti sticking coating" (such as polytetrafluoroethylene coating, ceramic coating) to reduce the adhesion of salt crystals, and even a small amount of crystals are easily washed away by the fluid.

Set up a dedicated slag discharge and flushing structure: Design a "conical slag discharge port" and an "online flushing interface" at the bottom of the evaporator, pipeline bends, and other areas where salt accumulation is likely to occur. After a certain period of operation, the concentrated salt slurry can be discharged in a timely manner through the slag discharge outlet to avoid salt accumulation; If there is a slight blockage, high-pressure water (or dilute acid solution) can be injected through the flushing interface to quickly clean the blocked area without stopping the machine for disassembly.

3、 Accurate control of operating parameters: avoiding salt supersaturation and precipitation

The key to preventing blockages is to control the concentration of wastewater within a safe range of "unsaturated but close to saturated" through real-time monitoring and dynamic adjustment of operating parameters

Real time monitoring of concentration and control of concentration factor: Install an "online refractometer" or "conductivity meter" at the outlet of the evaporator to monitor the concentration of wastewater in real time (such as TDS value). When the concentration approaches the saturation solubility of the salt (such as sodium chloride saturation concentration of about 26.5% at 25 ℃), start the slag discharge program in a timely manner to discharge the concentrated solution and avoid crystallization caused by concentration exceeding the saturation value. For example, when treating sodium chloride wastewater, the concentration endpoint can be controlled at 24% to 25% to ensure evaporation efficiency and reserve a safety margin.

Stable evaporation temperature to avoid local overheating: The core of low-temperature evaporation is to maintain a stable negative pressure and low temperature (usually 40-60 ℃) in the system. If the temperature fluctuates too much (such as a sudden increase in heat pump heating power), it will cause the local wastewater temperature to be too high and the salt solubility to temporarily increase. However, when the temperature returns to normal, supersaturated salt is prone to quickly precipitate and form crystal blockage. The heating power and vacuum pump negative pressure need to be stabilized through a PLC control system to control temperature fluctuations within ± 2 ℃.

Control the feed flow rate and uniformity: If the wastewater feed flow rate is too fast or the flow rate is unstable, it will cause the local wastewater in the evaporator to stay for too short (insufficient evaporation) or too long (excessive concentration), increasing the risk of crystallization. A "flow control valve" needs to be installed in the feed pipeline to stabilize the feed flow rate at 80% to 100% of the equipment's designed processing capacity, ensuring even distribution and stable evaporation of wastewater in the evaporator.

4、 Regular maintenance and auxiliary measures: delay the occurrence of crystallization blockage

By regular maintenance and auxiliary measures, trace crystals can be removed and blockage processes can be delayed to ensure long-term stable operation of the equipment

Develop a periodic cleaning plan: Based on the salt content of wastewater and equipment operation, develop a maintenance plan of "weekly small cleaning and monthly large cleaning". Small cleaning can use "online chemical cleaning", injecting low concentration citric acid solution (1%~3%) into the system, circulating for 1-2 hours, dissolving the calcium and magnesium salt crystals attached to the inner wall of the heating tube; For major cleaning, it is necessary to shut down the machine and use "high-pressure water jet cleaning" (pressure 30-50MPa) or "ultrasonic cleaning" to remove stubborn crystals in pipelines and valves and restore equipment flow.

Add specialized anti scaling dispersant: Add an appropriate amount of "polymer anti scaling dispersant" (such as poly (maleic anhydride), acrylic acid acrylic ester copolymer) at the wastewater feeding end. This type of agent can adsorb on the surface of salt crystals, inhibit crystal growth, and disperse small crystals in the wastewater to avoid their aggregation and adhesion to the equipment wall. Attention should be paid to selecting compatible chemicals based on the composition of the wastewater, and the dosage should be controlled at 5-10mg/L to avoid residual chemicals affecting subsequent treatment.

Regularly check the status of key components: check the operation status of components such as the circulation pump, slag discharge valve, and defogger every week - if the flow rate of the circulation pump decreases, it may be due to crystal blockage of the impeller, which needs to be disassembled and cleaned in a timely manner; If the defogger is blocked, it will cause an increase in the amount of liquid carried by the steam, and salt will enter the condensation system with the steam. At the same time, it will indirectly increase the risk of crystallization in the evaporator. It is necessary to regularly remove the defogger for flushing or replacement.