01 Introduction

The Ningxia Honing Chemical Zero Liquid Discharge Project was constructed by Luthuanghua Group Ningxia Honing Chemical Co., Ltd. It is located in the Coal Chemical Industry Park B area of Ningdong Energy Chemical Base, Yinchuan City, Ningxia Hui Autonomous Region, within the Ningxia Honing Chemical Co., Ltd. factory area, and is a new project.

This project has two wastewater streams: wastewater discharged after meeting standards from the wastewater treatment facility (mainly gasification unit wastewater) and concentrated brine from the desalination station (resin regeneration, reverse osmosis concentrate). The design treatment scale for wastewater reuse is 130m ³ /h, and the treatment scale for concentrated water from the desalination station is 150m ³ /h. The project is carried out in two steps: First, a large volume of low-salinity water is treated using membrane technology to reduce its volume; second, the high-salinity water after volume reduction is subjected to evaporation crystallization. The mixed salt from evaporation crystallization is sent out for processing, and the entire project ultimately achieves "zero liquid discharge" of wastewater.

Honzon Think (Beijing) Environmental Engineering Co., Ltd. (hereinafter referred to as Honzon Think) is responsible for the process package design, complete equipment supply, and technical services for the high-salinity wastewater evaporation crystallization process section, achieving zero liquid discharge of wastewater throughout the plant.

1 Evaporation Crystallization Unit Design Water Volume and Water Quality

1.1 Design Influent Water Volume and Water Quality

The influent to the evaporation crystallization unit is concentrated brine produced after volume reduction by the membrane system. The qualified condensate water produced during the evaporation process is sent to the recycled water tank, and the concentrated liquid is treated by crystallization to obtain mixed salt, ultimately achieving zero liquid discharge.

The normal treatment scale of this project is 10m ³ /h (average continuous flow), and it can operate stably under an operating flexibility of 50%~110%.

The influent water quality of the evaporation crystallization unit is shown in Table 1:

Table 1 Influent Water Quality Parameters of the Evaporation Crystallization Unit

1.2 Design Product Water Quality

Table 2 Condensate Water Quality

1.3 Mixed Salt Moisture Content and Concentrated Mother Liquor

Under the above design water quality and design scale, the moisture content of the mixed salt after separation by the dewatering machine is ≤20%.

Under normal circumstances, there is no wastewater discharged externally. In special cases, a small amount of concentrated mother liquor from evaporation crystallization is discharged externally. The concentrated mother liquor is sent to the incinerator in the plant for incineration, and the discharge amount of the concentrated mother liquor is ≤0.5 m ³ /h.

2.1 Process Explanation

Figure 1 3D Model of the MVR Unit

Combining the characteristics of water quality and quantity and energy conditions (electricity price, steam price) of this project, Honzon Think designed and selected an MVR forced circulation evaporation crystallization system. Through evaporation and concentration, the high-salt wastewater is concentrated to supersaturation until salt crystallizes out, and then centrifugal dehydration is used to obtain mixed salt. A small amount of mother liquor is discharged to the outside for treatment to reduce the impact of accumulated impurities on the stable operation of the MVR unit.

The basic principle of MVR is to compress the secondary steam that would normally be condensed with cooling water in an evaporator using a compressor to increase its pressure and saturation temperature, and then send it to the evaporator heater as a heat source to heat the material liquid. The latent heat of the secondary steam is fully utilized, thus achieving energy saving.

Compared with traditional evaporators, Honzon Think's MVR evaporator has the following advantages:

(1) The special design of MVR can adapt to complex influent water quality and a wide range of water quality changes, ensuring the continuous and stable operation of the MVR system.

(2) High compression ratio, high thermal efficiency, energy saving, low specific energy consumption. The energy consumption for evaporating 1t of clear water is about 1/6～1/5 that of traditional evaporators (energy consumption varies with different materials), and the operating cost is greatly reduced.

(3) Intelligent, the various operating parameters of the compressor can be monitored through software, and analysis reports can be generated.

(4) The compressor uses frequency conversion control, and in actual use, it can automatically perform frequency conversion control according to the on-site situation.

(5) The compressor system has multi-directional automatic protection functions, and automatically performs protection when changes occur in temperature, pressure, etc., ensuring the normal use of the compressor.

(6) Since the heater is also a condenser for secondary steam, no separate condenser is needed, and there is no consumption of circulating cooling water.

(7) Small footprint, fewer operators, and fewer supporting public works projects.

2.2 Process Description

The process flow diagram of the MVR unit in this project is shown in Figure 2. According to the process flow, it can be mainly divided into a preheating system, an evaporation crystallization system, a centrifugal dehydration system, and a steam condensate system.

Figure 2 Process Flow Diagram

This project was completed and commissioned in January 2021. The MVR unit achieved the design index requirements upon its first startup and can operate continuously and stably.

Figure 3 On-site Equipment and Salt Output

After the commissioning of the MVR unit for the Ningxia and Ninghe Chemical Liquid Zero Discharge Project, the concentrated brine produced by the front-end system can be separated, and the evaporated and crystallized mixed salt can be sent out for processing. The condensate water is used in the production system instead of fresh water, achieving zero liquid discharge and resource utilization. This not only reduces water pollution and improves economic efficiency but also reduces water consumption, achieving "zero liquid discharge" of industrial wastewater.