Abstract:Mineral characteristics, sedimentary structures and formation mechanism of scabs produced in high-pressure digestion and evaporation process were analyzed by X-ray diffraction(XRD), Backscatter electron(BSE) image and electronic energy spectrometer(EDS). The results show that scab formation could be divided into early and later sedimentation respectively. The scabs generated in high-pressure dissolution are chemical crystallization including coarse crystalline hematite, perovskite and goethite, which grew toward center along tubes walls and formed a large number of dense and coarse clusters growing as perpendicular to the wall. Then the solution captured between crystals recrystallized to form fine alumogoethite, sodium-silicon residue, magnesium hydroxide, calcium magnesium silica slag, hydrated garnet and peridot, and existed in form of intergranular interstitial material. Therefore, early deposition is the main scab process in high pressure dissolution process. Formation of scabs in evaporation process is physical deposition that occurs when a large number of crystallizable impurities are reduced. Firstly, loose and fluidity fine particles or aphanitic sodium carbonate and sodium-silicon residue substrates are formed by deposition, and then calcium iron titanate, iron garnet, sodium carbonate and other crystals are precipitated and form scab. Evaporation process is dominated by late deposition.