Process Optimization and Quality Control of a Combined Impurity Removal–Crystallization Process: Preparation of High-Purity Silver Nitrate for Photovoltaic Back Silver Applications

The printing adaptability and sintering compactness of photovoltaic back silver paste are highly dependent on the physical properties of the silver powder, such as tap density and particle size distribution, which in turn are directly influenced by the purity of the precursor silver nitrate. When using 4N silver ingots as the raw material, trace impurities such as Nb, Fe, Cu, and Pb remaining in the crude solution can seriously interfere with the uniform nucleation and growth of silver crystallites, leading to decreased tap density and broadened particle size distribution. This paper presents a combined impurity removal–crystallization process integrating an adsorbent with controlled evaporative crystallization. Under optimized conditions, the concentrations of Nb, Fe, Cu, and Pb impurities decreased from 44, 21, 16, and 12 ppm to below 4 ppm, respectively. Meanwhile, the controlled crystallization step further enhanced the purity of the silver nitrate crystals through the redistribution of impurities between the solid and liquid phases. When the high-purity silver nitrate prepared by this process was used to produce photovoltaic back silver powder, the tap density of the resulting silver powder increased from 2.5 g cm−3 to 3.4 g cm−3, accompanied by a smooth surface morphology. Thermogravimetric (TG) analysis further confirmed a lower residual organic content on the particle surface. These comprehensive improvements in powder properties demonstrate the effectiveness of the proposed process in producing high-quality silver powder for photovoltaic back silver applications.