A controllable vertical temperature gradient enhances secondary desalination in process-integrated continuous freeze desalination.

Conventional freeze desalination suffers from discontinuous post-treatment and insufficient secondary desalination efficiency. To tackle these issues, this study develops a process-integrated freeze desalination system with a split-type silicone oil layer, where a precisely controlled vertical temperature gradient enables in-situ coupled ice buoyancy, surface melting and brine separation during ice ascent. Orthogonal experiments demonstrate that the mid-upper layer temperature is the dominant factor affecting desalination rate (range R = 28.715, p < 0.001). Under optimal gradient configuration, the desalination rate reaches 55.00%-68.57% for 20-35 ppt feedwater, corresponding to an improvement of 28-31 percentage points over the non-melting baseline. Compared with the uniform-heating buoyant desalination process, this gradient-controlled scheme reduces desalination performance fluctuation across 20-35 ppt feed salinity by ∼18% and eliminates ice suspension failure risk within the tested temperature range, providing quantitative guidance for efficient and stable secondary desalination enhancement.