1. Advantages of freeze-drying sludge pretreatment
(1) The entire freeze-drying process is carried out at low temperature to avoid the loss of volatile substances such as mercury;
(2) The entire freeze-drying process is carried out under closed conditions to avoid environmental pollution;
(3) The sludge after drying is loose and porous, which is convenient for grinding and processing;
(4) The freeze-drying method is highly efficient and can freeze-dry more than 20 samples in 16 hours;
(5) The freeze-drying operation steps are simple and the freeze-drying process does not require personnel to be on duty. It can be carried out at night and does not affect daily testing work.
2. Introduction to the freeze-drying sludge pretreatment process
(1) Pre-freezing: When the product is freeze-dried, it needs to be placed in a suitable container and then pre-frozen before sublimation drying can be carried out. The pre-freezing process is not only to protect the main properties of the material from changing, but also to obtain a reasonable structure of the frozen product to facilitate the sublimation of water, and to have an appropriate loading volume for future application. The product is divided into containers with a certain surface area to thickness ratio. The surface area should be larger and the thickness should be smaller. A large surface area is conducive to sublimation, while a large product thickness is not conducive to sublimation. Generally, the packaging thickness is no more than 10 mm. Pre-freezing can be done in an ultra-low temperature refrigerator.
(2) First stage desorption drying. This stage is mainly sublimation drying. It is generally believed that the appropriate pressure of the freeze drying chamber is between 0.1 and 0.3 mbar. Within this pressure range, it is conducive to both heat transfer and sublimation.
(3) Second stage drying. Once the ice in the product has sublimated, the drying of the product enters the second stage. Although there is no frozen ice in the product at this stage, there is still about 10% water in the product. In order to make the product reach the qualified residual moisture content, the product must be further dried. At this stage, the temperature of the product is rapidly raised to the maximum allowable temperature of the product and maintained at this temperature until the freeze drying is completed. Rapidly raising the product temperature is conducive to reducing the residual moisture content of the product and shortening the desorption drying time. The allowable temperature of the product depends on the type of product.
3. Conclusion
(1) The freeze drying method can significantly shorten the sample testing cycle and will not cause the loss of volatile elemental mercury.
(2) Freeze-dried and ground sludge is more uniform and the sample is more representative.
(3) Determination of heavy metals in sludge after freeze-drying can avoid the result deviation caused by moisture content.
(4) For wet sludge, there is no need to measure the moisture content separately, which can reduce the workload.