| The tin, lead and silve in the Sn-Pb-Ag alloy which is intermediate in the fire refining of crude tin, are obtained by cooperation vacuum distilation with crystallizer. After Sn-Pb-Ag alloy is distilled in vacuum ambiance, however, the silver with high boiler stays in residue-crude tin, which is sent to crystallizer. Although this technology has been applied to actual production, it still has some trouble, such as the low production efficiency and high labor intensity. So desilverization by zinc addition may provide new idea for fire refining of crude solder before crude solder sent to vacuum furnace, and make the fire refining of crude tin abundant. The liquidus structure of alloy and phase equilibrium in liquid which have an important effect on desilverization by zinc addition, so it is very important that the structure of liquidus alloy which is calculated by molecular dynamics and drawing three-dimensional phase diagram.Firstly, The liquidus surface of three-dimensional phase diagrams of Sn-Pb-Ag were obtained with the method of invoking interpolation-function (griddata) programing which was interpolation-method and the method of invoking regress-function (regress) programing which was fitted surface-equation-method, respectively, in MATLAB. The results show that the liquid surface drawn by fitted surface-equation-method is smoother than that by interpolation-method, at the same time it can clearly shows that the crystallization-line and eutectic point. The determination-coefficient of surface equations of (Ag)、Ag5Sn、Ag3Sn、(Pb)、(Sn) phase arel.000、0.9819、0.9855、0.9872、0.9713, which indicate the credibility of surface equations. In order to further examine the accuracy of the liquid surface, the data of isotherm in the Sn-Pb-Ag phase diagram were chosen to compare its original values with fitted values. The results show that the maximum deviation is23.67℃, the minimum deviation is0.49℃,deviation absolute value-average is6.57℃, relative error-absolute value-average is1.36%. Original values are consistent with fitted values. The fitted surface-equation-method is trustworthy. Comparing with Sn-Pb-Ag two-dimensional projection, three-dimensional phase diagram of Sn-Pb-Ag can display temperature of any point on liquid surface and temperature changing trend of liquid surface. Secondly, the liquidus structure of solder binary alloys and ternary alloys were calculated by Ab-initio molecular dynamics at450℃. The results of binary alloy calculation show that the diffusion coefficients of Sn-Ag, Sn-Zn, Sn-Pb et al binary alloys are DSn-Zn>DSn-Pb>Dsn-Ag So the force between Sn atom and Ag atom is strongest, and the force between Sn atom and Zn atom is weakest. Moreover, Sn-Zn and Sn-Pb liquid alloy only are short-range order. Sn-Ag liquid alloy not only is short-range order, but also is medium-range order. It drew conclusions that Sn-Zn and Sn-Pb alloy were liquid at450℃, and Sn-Ag alloy was partial meltdown. It may arise from that there are two intermetallic compounds with higher melting point, ζ, and ε phase, in Sn-Ag system. The results of ternary alloy calculation show that the energy band of Sn-Pb-Ag, Sn-Ag-Zn, Pb-Ag-Zn et al ternary alloys are continuous at Fermi level, and all of ternary alloys are metallic state. Through Ab-initio molecular dynamics calculation, there is the bonding of Ag atoms and Sn atoms, and the average distance between Ag atom and Sn atom may increase0.251A than that of origin, so they may product charge transfer compounds in Sn-Pb-Ag system. There are four Zn atoms-Zn1, Zn2, Zn3and Zn4around Ag1atom, which are not bonding with Ag1atom in original structure of Sn-Ag-Zn. After the calculation of Sn-Ag-Zn system, Ag1is bonding with Zn1and Zn2. So Ag atom is likely to bond with Zn atom in Sn-Ag-Zn system. The distance between Ag1atom and Zn1atom is2.475A, but the distance between Ag1atom and three Pb atoms are3.434A,3.291A and3.571A, respectively, in Sn-Pb-Ag system. So there is intense interaction between Ag atom and Zn atom, and tendency of Ag-Zn intermetallic compounds production in Sn-Pb-Ag system.Finally, the research on the thermodynamic of reaction of Ag and Sn or Zn, the content of Ag in the dross, and the composition, micro structure and distribution of Ag-Zn intermetallic compound (IMC) were carried out using SEM-EDS and chemical composition analysis after Zn was added into the Sn-36.6wt.%Pb-lwt.%Ag system. The results were compared with that of Pb-1wt.%Ag system with addition of Zn. It was found that the formation of Ag-Zn IMC was easier than that of Ag-Sn IMC, the content of Ag in the dross was less than1%, three phase regions:the phase region of rich tin (Sn-12.1wt.%Pb-0.52wt.%Ag-2.42), the phase region of rich lead (Pb-31.34wt.%Sn-1.89wt.%Zn) and Ag-Zn IMC (Zn-27.3wt.%Ag-10.87wt.%Sn-3.7wt.%Pb) were distributed evenly in the Sn-Pb-Ag-Zn system. There were only two phase regions, however, in the Pb-Ag-Zn system:the white phase region with99.45%Pb at the bottom of the alloy and the grey Ag-Zn IMC phase region on the upside. There was a transition region between bottom and upside of Pb-Ag-Zn system. The closer to the bottom of transition region, the less content of Ag-Zn IMC will be. The reasons for these phenomenon could be attributed to:(1) the density of Sn-36.6wt.%Pb-1wt.%Ag was only0.35g/cm3larger than that of Ag-Zn IMC;(2)Sn-36.6wt.%Pb-1wt.%Ag was rich tin-based system in which liquid had not dual-phase stratified region. Nevertheless, Pb addition to Sn-Pb-Ag led to the density of system increase and dual-phase stratified region existence in liquid phase with a lot of lead, which made enrichment of silver in Sn-Pb-Ag possible by Zn addition. |
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