Analysis of the reasons for unsatisfactory separation performance of the centrifuge

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Analysis of the reasons for the unsatisfactory separation performance of the centrifuges. The recycling workshop of the acrylic fiber factory is equipped with two centrifuges, both of which are horizontal spiral discharge sedimentation centrifuges. One of them was introduced from Japan along with the entire set of equipment when operations started in 1988, while the other was imported from Japan in May 2001. Both were manufactured by the Japanese company TANABE, with their core technology based on that developed by the West German company Flösswegwerk. 　　The main function of the recycling workshop in the entire acrylic fiber plant is to remove impurities and concentrate the sodium thiocyanate solution; subsequently, the sodium thiocyanate solution with a concentration of 56% is sent to the polymerization workshop for use in preparing the spinning masterbatch. In the material circulation of the entire acrylic fiber production facility, sodium sulfate and sodium thiocyanate circulate together. Nearly 90% of the sodium sulfate is separated from sodium thiocyanate during the crystallization and separation step, and the separation of sodium sulfate relies on centrifuges. Therefore, in the entire recovery system, centrifuges play a crucial role, just like the kidneys in humans, which remove impurities to ensure clean blood flow. The efficiency of the centrifuges’ separation process directly affects the stable operation of the entire facility and even the whole plant over the long term. 　　The principle behind separating sodium sulfate during the crystallization process is as follows: Since sodium sulfate has a very low solubility in sodium thiocyanate solution and tends to form crystals easily, the concentrated sodium thiocyanate solution is allowed to settle in a sedimentation tank, allowing the sodium sulfate crystals to grow gradually at the bottom of the tank (with a sodium sulfate content of about 13%–15%). The function of the centrifuge is to separate the sodium sulfate crystals from the liquid at the bottom; only a small portion of these crystals is sent for evaporation in a five-effect evaporator to be used as seed crystals. The solid phase that is separated is dissolved in a dissolution tank, and the resulting wastewater is sent to a treatment unit where resin is used to replace the sodium thiocyanate contained therein, after which the waste liquid is discharged. If the separation efficiency of the centrifuge is poor, the sodium thiocyanate content in the solid-phase separation product becomes too high, exceeding the adsorption capacity of the resin used for treating wastewater. Moreover, environmental regulations require that the sodium thiocyanate concentration in discharged wastewater not exceed 200 ppm; as a result, the plant is forced to pour this sodium thiocyanate solution with a high concentration of sodium sulfate back into the system in order to concentrate it and recover it. Over time, the sodium sulfate concentration in the system continues to rise, which in turn causes fluctuations in the operation of the evaporation and crystallization systems, thereby affecting the normal production of acrylic fiber facilities. 　　Based on the above separation principles, it can be seen that in recovery devices, the effectiveness of centrifugation depends on the degree of dryness of the solid phase; in other words, the better the separation effect, the less sodium thiocyanate contained in the separated solid phase. Below, we will start by examining the working principle of centrifuges, analyze the reasons for suboptimal separation effects, and seek corresponding solutions. 1) Working principle: The horizontal spiral discharge centrifuge is used in recovery systems for the solid-phase concentration of suspensions. It has a column-cone configuration and consists of two independently rotating rotors – one is a cylindrical-conical, pore-free drum, and the other is a screw with spiral blades; these two components are mounted coaxially and horizontally together. The drum of the centrifuge rotates counterclockwise, while the spiral inside it rotates in the same direction and concentrically with the drum; the speed of the drum is slightly higher than that of the spiral. When the suspension is introduced into the spiral cavity through the feed pipe and enters the sedimentation zone (the cylindrical part) of the drum via the distribution port, under the action of centrifugal force, the solid particles with higher specific gravity settle on the inner wall of the drum. These particles are then pushed out of the sedimentation zone by the spiral blades and discharged from the small end of the drum through the drying zone. The clarified separated liquid flows along the channels provided by the spiral blades to the large end of the drum and is discharged through the overflow holes; in this way, the solids and liquids in the suspension are separated from each other.