“Tank‑within‑Tank” Cyclone Separation Technology Solves the Challenge of Semi‑Coke Wastewater Treatment: A New Pathway for Efficient Separation and Resource Recovery

Application Background

Semi‑coke wastewater is generated during the low‑to‑medium temperature dry distillation (carbonization) of coal (at approximately 650°C). Because the process involves low‑temperature distillation, it yields significant quantities of tar and a high content of low‑molecular‑weight organic matter. Consequently, the wastewater contains high concentrations of tar and low‑molecular‑weight organics, with pollutant levels roughly ten times higher than conventional coking wastewater. This makes biological treatment extremely difficult. Without effective physicochemical pretreatment, the heavy tar and organic pollutants will poison microorganisms in subsequent biochemical tanks, leading to system failure.

Traditional semi‑coke wastewater pretreatment relies on multi‑stage settling tanks and oil skimming pools. These conventional processes require large land occupation, long hydraulic retention time, and cannot efficiently capture fine tar droplets. Many coal chemical plants are searching for compact, high‑efficiency pretreatment solutions to recover coal tar while reducing pollutant loads.

The Tank‑within‑Tank cyclone separation system provides an integrated solution for semi‑coke wastewater treatment and hazardous liquid storage, featuring a dual‑layer structure. For separation applications, the outer tank provides flow equalization and buffering, while the inner tank incorporates hydrocyclones, oil skimmers, and settling hoppers to achieve efficient three‑phase (oil‑water‑sludge) separation, reducing footprint by over 40%. Tank‑within‑Tank oil water separator can effectively intercept coal tar and suspended sludge from semi‑coke wastewater, realizing resource recovery of tar products. For projects that need to handle plenty of free oil and dispersed oil before biochemical units, users can also consider CPI corrugated plate coalescing oil separator as matched pretreatment equipment.

Working Principle of Tank‑within‑Tank Cyclone Separation Technology

The core of this integrated technology combines hydrocyclone centrifugal separation and gravity sedimentation inside a nested tank structure.

Flow Route of Wastewater Inside the Tank

When semi‑coke wastewater enters the outer tank, the outer tank stabilizes fluctuating inflow flow and equalizes water quality. Then the wastewater flows into the inner tank assembly. Under centrifugal force generated by hydrocyclones, heavy sludge is thrown toward the wall and settles down to the bottom hopper. Fine tar droplets with lower density move toward the center and rise upward. An integrated oil skimmer continuously collects the separated tar.

The whole unit completes three‑phase separation of oil, water and sludge inside one compact tank body. No extra large concrete pools are required.

Dual‑layer Tank Safety & Monitoring Design

The interstitial space between inner and outer tank is equipped with intelligent monitoring sensors for leak detection. For storage scenarios, the inner corrosion‑resistant layer holds the medium, and the outer high‑strength shell offers leak protection with intelligent monitoring in the interstitial space. This equipment holds Chinese invention patent ZL 2024 1 0054265.3, targeting floating oil and large dispersed oil particles above 50‑100 μm.

Core Advantages for Semi‑Coke Wastewater Treatment

1. High tar removal & resource recovery
It efficiently captures both large tar lumps and fine emulsified tar droplets, recovering coal tar as recyclable raw materials instead of discarding it as waste pollutants. The collected oil features high purity and low water content to improve oil resource utilization.

2. Small footprint & modular design
The tank‑within‑tank integrated structure cuts land occupation by more than 40% compared with traditional multi‑stage sedimentation tanks. It is suitable for reconstruction projects of existing coal chemical plants with limited space. Multiple specifications from 10 m³/h up to 1000 m³/h are available for selection.

3. Low maintenance & long service life
The system features fully automated control with real‑time data feedback. Operators only need periodic discharge of oil and sludge. The closed‑container design prevents odour escape and secondary pollution, supporting long‑term continuous unattended operation.

4. Strong anti‑corrosion performance
The inner tank can be customized with anti‑corrosion lining materials, adapting to corrosive phenolic wastewater from semi‑coke production. Operating temperature ranges from 5‑80℃, with steam heat‑tracing option (0.4 MPa).

Industrial Application & Effect

In coal chemical semi‑coke projects, this tank‑within‑tank cyclone pretreatment unit is installed before biochemical treatment. After treatment, most tar and suspended solids are removed. The organic load into the biochemical system is greatly reduced, protecting biochemical bacteria from toxic phenolic and tar pollutants. Outlet water oil content can reach ≤150 mg/L (excluding emulsified and dissolved oil).

Besides semi‑coke wastewater, this integrated system can also be applied in petroleum, chemical, food, and pharmaceutical industries, supporting customized designs in volume and materials (carbon steel, stainless steel, anti‑corrosion coating).

Conclusion

Semi‑coke wastewater treatment remains a tough problem in coal chemical industry due to high tar and organic pollutant concentration. The tank‑within‑tank cyclone separation technology combines hydrocyclone, gravity settling and oil skimming in one compact nested tank. It achieves efficient three‑phase separation and coal tar resource recovery. As a reliable physicochemical pretreatment solution, it offers a new pathway for coal chemical enterprises to reduce wastewater treatment cost and improve resource utilization.