Refining by fractional melting

Abstract

A new and highly effective method of purifying metals is described. The process, a fractional melting process, involves heating an alloy within its liquid-solid region while simultaneously compressing it against a filter to remove the interdendritic liquid. A simple mathematical model is developed_which shows that remarkably high “refining ratios”, ¯C_c/C_o, can be obtained (where ¯C_c is wt pct solute of the refined “cake” and C_o is initial wt pct solute). For example, for a “yield” of refined solid of 0.4 of the original sample and a partition ratio,k, of 0.1, this new process achieves a refining ratio of as low as 10⁻⁴, 1000 times lower than isothermal separation and comparable to that obtained by multi-pass zone refining. Refining ratio is shown to depend on partition ratio, “instantaneous cake wetness” during the refining process, final fraction solid and extent of diffusion in the solid. (“Instantaneous cake wetness” is defined as fraction liquid in the cake at any given instant during refining.) Refining experiments were conducted on a series of Sn-Pb alloys in which samples were heated under a pressure of 21 MPa (2900 psi), over a temperature range of 5 to 20°C above the eutectic temperature. Refining ratios obtained are in agreement with theory, assuming low “instantaneous cake wetness”. Calculations showed this wetness to be about 0.02. Final structures of the refined solid are fine grained and completely devoid of second phase.