In the production process of evaporation crystallization of salt containing materials, salt legs are usually set at the bottom of the evaporation crystallizer. The salt legs play the roles of salt collection, classification, washing, re dissolving soluble impurities, cooling and transportation. They are one of the key components to improve the quality of salt and reduce the energy consumption of the salt making system. The role of salt legs should not be underestimated. In recent years, with the revision of China’s new environmental law and the strengthening of environmental protection supervision, the requirements for high salt wastewater treatment are divided into strict. Many production wastewater require zero discharge treatment. Zero discharge treatment often involves evaporative crystallization process, and the design of salt leg is an important link of evaporative crystallizer. Exploring salt leg technology is of great significance for the purification of crystalline salt from salt containing wastewater.
Working principle of salt leg design
The design of salt leg uses its specially designed internal structure to provide appropriate speed and uniform fluidization, which can efficiently elutriate, cool, classify and thicken.
The salt crystals continuously crystallized in the evaporation crystallizer move downward due to gravity, and gradually fall from the cone to the salt leg. The raw material liquid enters from the lower part of the salt leg, rises along the leg wall and washes against the salt crystal, bringing gypsum, fine sodium sulfate and salt particles into the evaporation crystallizer to continue to participate in circulation and growth; At the same time, soluble impurities can be dissolved and removed, which is conducive to the improvement of salt quality; Heat exchange can also be carried out between the feed liquid and the reverse running salt slurry to reduce the salt discharge temperature and reduce the heat loss of salt discharge.
Salt leg form currently used in China
At present, there are three types of salt legs commonly used in China. The salt legs shown in Figure 1-A are mainly used in the salt making device with gypsum brine as raw material and produced by gypsum seed method, which is still more practical in the transfer salt discharge process. The salt leg shown in Figure 1-B is mainly used in the salt and nitrate cogeneration and salt making unit with mirabilite brine as raw material and Sulzer and massol technologies. The salt leg shown in figure 1-C is a salt making unit with HPD technology, and the raw material of the unit is purified gypsum brine. In this salt leg design, an inverted cone cylinder is added inside, the structure is relatively more complex, and the equipment manufacturing is relatively difficult. Once the structural design is unreasonable, the inverted cone cylinder will also fall off, resulting in unnecessary losses.
The above three salt leg forms are all designed with a elutriation brine pipe. Fig. 1-A is provided with a ring pipe, which is located below the straight section of the salt leg. Figure 1-B and figure 1-C are respectively provided with two and three independent elutriation brine pipes, one on the cone, and the rest are set at the lower position of the straight section of the salt leg. The function of salt leg elutriation brine is the rational application of “crystallization kinetics method” and “fluidization technology”. The elutriation brine cools the salt slurry discharged from the salt leg, washes the impurities of the product crystal, and returns the fine crystal to the evaporation chamber.
Precautions for salt leg design
The salt leg set at the bottom of the evaporator plays many roles, such as salt collection, classification, washing, re dissolving mirabilite, transportation and so on. It is a key component to ensure salt quality. The working condition in the elutriation leg is like a countercurrent fluidized bed. In addition to the rising speed of the elutriation brine and the settling speed of the salt crystal are the main factors affecting the operation, the following items shall also be considered in the design:
- The pressure and flow of elutriation brine shall be stable, and its flow shall be adjusted with the change of salt slurry discharged.
2 the temperature of the elutriation brine shall not only reduce the heat consumption, but also prevent the precipitation of mirabilite, so as to avoid the scaling of the flowmeter and pipeline and affect the production.
- There shall be a salt layer with sufficient height in the salt leg to ensure that the elutriation brine rises evenly to prevent gully flow.
4 eliminate the interference of large salt.
5 control the temperature of elutriation water at about 53 ℃.
- Inspection hole shall be considered for the cone part of salt leg.