Application of double paddle combined with disc drying in the production of precipitated barium sulfate

This paper introduces the combined drying process, equipment structure and working principle of double paddle blade and disc dryer system, analyzes and compares the energy consumption and environmental friendliness of the combined drying system, and affirms the rationality and obvious energy saving effect of the combined drying system compared with the traditional flash and convection drying combination in industrial barium salt production.

This paper introduces the combined drying process, equipment structure and working principle of double paddle blade and disc dryer system, analyzes and compares the energy consumption and environmental friendliness of the combined drying system, and affirms the rationality and obvious energy saving effect of the combined drying system compared with the traditional flash and convection drying combination in industrial barium salt production.

BaSO4 is a colorless orthorhombic crystal or amorphous white powder. It is nontoxic and decomposes at 1450 ℃. Its chemical properties are stable, and its solubility in water is very low (the solubility in water at 60 ℃ is 0.46g/100g water), which is mainly used for paint filling. In the process of barium sulfate, drying is the last process. At present, there are many kinds of drying methods commonly used in China, including rotary flash drying, two-stage double blade drying, rotary drying kiln drying, double blade plus disc combined drying. The production mode of drying is directly related to the final quality of the product, affecting the final moisture content, whiteness and other technical indicators of the product. We designed, manufactured and installed a precipitation barium sulfate system with an annual output of 40000 tons (by-product 22000 tons of sodium sulfide) for an enterprise. The combined drying system with double paddle blades and disc is adopted. The moisture content of barium sulfate slurry is about 25%, and the material is white paste. After final drying, the moisture content of the material should reach 0.2%, and the apparent density should be 3.5-3.8kg/l.

1. Process flow and equipment

1.1 process selection

During the drying process from 25% water content to 10% water content, the physical properties of barium sulfate slurry gradually become sticky and hard, and then become loose. The double blade dryer is suitable for this change. The material with water content less than 10% is loose and has good fluidity, so the disc dryer is selected. Therefore, in this project, the combination of double blade dryer and disc dryer is adopted. Both of them belong to conduction type dryer. The technological process is as follows: after centrifugation by horizontal spiral centrifuge, the wet materials with water content of about 25% are first put into the double paddle dryer, pre dried to about 10% moisture content, and then enter the disc dryer for drying operation (the process is shown in Figure 1), and finally meet the product moisture requirements.

1.2 double blade dryer

The double blade dryer is a conduction dryer. The machine body is composed of jacket shell, double screw drying and conveying shaft, driving mechanism, top cover, and dehumidification system. The inner heating double screw conveying dryer works horizontally, the shell jacket and the drying conveying shaft are filled with 0.6MPa steam. The wet materials are added into the dryer from the feeding port on the top cover of the dryer. Driven by the rotating drying conveying shaft, the materials continuously roll forward, and the materials complete the heat transfer drying operation in the process of movement; after the drying is completed, the bottom outlet at the other end of the dryer is discharged, and At the same time, the conveying process is completed.

1.3 features of double blade dryer

(1) the double paddle dryer can handle a wide range of materials, including paste, granular and powder materials, which can handle both heat sensitive materials and materials requiring high temperature treatment; (2) the whole drying process is closed operation, which makes the material turn violently during mixing and mixing, and the material particles fully contact with the heat transfer surface, and the material temperature is within the axial range The gradient of humidity and mixing degree is very small, so as to ensure the stability of the process; at the same time, it also obtains high heat transfer coefficient, which can reach 80-350w / ((M2 · K) generally; (3) the double blade dryer indirectly heats the materials without a lot of air to carry away the heat, and the energy consumption is relatively low; (4) the water vapor released from the materials in the dryer is directly from the exhaust pipe at the top of the dryer The main components of the tail gas are steam, low gas speed and less dust, so there is no entrainment problem of drying products; the environmental pollution is small, and the investment of dust recovery equipment is reduced; 5) the double blade dryer can be operated continuously or intermittently, with low speed mixing and reasonable structure. The wear is small and the maintenance cost is very low.

1.4 disc dryer

The disc continuous dryer is a new type of conductive drying equipment. It works vertically and is composed of drying disc, rake arm, a number of rake leaves installed on the rake arm, main shaft, transmission device, shell, etc.; the wet material is added by the feeding port at the top, and the hollow drying plate is filled with heat transfer oil of 250 ℃. The drying plate is divided into large plate and small plate. The drying plate is arranged horizontally in the dryer. The drying plate is stationary. The material moves from the inside edge of the large plate to the inner edge of the small plate under the push of the rake blade. The material moves from inside to outside on the small plate, and falls into the large plate from the outer edge of the small plate. The material is transported and turned on the surface of the drying disk by the trailing leaves, which is Archimedean spiral track The trace movement is heated by the drying plate in the process of movement for heat and mass transfer to complete the drying operation. After passing through each layer of drying plate, it is discharged from the bottom of the dryer. The whole drying process is closed. The water vapor released from the material is discharged to the atmosphere by the induced draft fan after being removed by the pulse bag filter.
Features of 1.5 disc dryer

(1) in the heating disk, the material is pushed by the rake blade slowly, so the mutual wear between the materials is very small, the shape and size of the material before and after drying change very little, the damage rate is very low, almost no influence on the surface quality of crystal material; (2) the material temperature control is accurate and easy, and the residence time of the material can be adjusted; (3) the flow direction of the material is single, there is no backmixing phenomenon, and the drying is uniform (4) the disc continuous dryer is simple in configuration and does not need to be equipped with cyclone dust collector which is necessary for air flow drying, and does not use hot air as heating medium. Therefore, it will not cause product loss due to poor separation of hot air and dust along with tail gas; it can operate continuously, operate in closed air, with high evaporation intensity, reasonable humidity gradient distribution and power Low consumption and small floor area;

1.6 equipment selection

As BaSO4 products have strict requirements for iron ions, 0Cr18Ni9 stainless steel is used for the contact parts of double paddle dryer and disc dryer; the supporting facilities, such as dust removal equipment, are also made of 0Cr18Ni9 stainless steel in order not to affect the quality of recovered products.

2.Equipment performance
The double paddle leaf dryer and disc dryer are combined to dry barium sulfate, which fully and effectively utilizes their respective advantages. The double paddle dryer effectively solves the problem of wet barium viscosity, which is not easy to deal with; the disc dryer effectively solves the problem of deep drying with moisture below 10%, and the combined system has a good effect in the industrial production process.

Compared with flash drying system, the combined drying system with double blades and trays can save about 30% - 40% energy under the same production and production conditions, but the cost of the combined drying system with double blades and trays is higher; compared with the two-stage double blade drying system, the cost and power consumption of the latter are higher; the combined drying system with double blades and trays can save about 30% - 40% energy Compared with rotary drying kiln, the latter system occupies a larger area, has a higher investment, and the whiteness of the product can not be guaranteed.