Study on dechlorination salt characteristics of pickling sludge by a water-washing process

Steel hydrochloric acid pickling sludge (SHPS), containing the heavy metals Fe, Zn, and Ni and a high chloride salt content, is considered a hazardous solid waste. With the gradual reduction of high-grade metal mineral resources such as Fe, Zn and Ni, it is particularly urgent to recycle valuable metals such as Fe, Zn and Ni in solid waste SHPS in order to realize the resource utilization of SHPS and reduce the environmental harm caused by SHPS. In addition, SHPS usually contains different amounts of alkali chloride, which will have a serious adverse impact on the subsequent extraction and smelting process of Fe, Zn and other metals. Therefore, the removal of chloride plays an important role in the resource utilization of valuable metals in SHPS. Thus, in this study, the effects of water washing dechlorination process parameters such as liquid-solid (L/S) ratio, SHPS particle size, washing time and washing frequency on the chloride removal rate were investigated. The best experimental parameters of SHPS washing were obtained. At the same time, the microscopic morphology and crystal phase composition of SHPS before and after washing were explored. The results showed that the optimized conditions were as follows: room temperature, a L/S ratio of 3 : 1, an SHPS particle size of 100 mesh, and 10 min of water washing, repeated two or three times; under these conditions, the removal rate of Cl, Na, Ca, K, Mg, and S reached 96.64-99.68%, 97.38-99.89%, 36.40-60.37%, 49.11-54.82%, 39.18-40.22%, and 36.98-42.13% respectively. The contents of Cl, K, and Na in filter residue (FR) meets the requirements in GB/T 36144-2018 and GB/T 32545-2016. Conversely, the contents of Fe, Zn, Mn and Ni in the FR are enriched, which is more conducive to the subsequent resource utilization of SHPS. The scanning electron microscope (SEM) image shows the particle size of the FR particles is reduced after washing. The X-ray diffractometer (XRD) results proved that the chlorine salt content in the FR after washing was significantly reduced, the diffraction peaks of Al₂O₃ appeared in the FR, and the diffraction peak intensity of CaCO₃, Fe₂O₃ and SiO₂ increased.