Solubility of CaF_2 in NaCl-KCl salt flux for Al-recycling and its effect on Al-loss

摘要

CaF_2 is often used as a fluoride addition to chloride base salt fluxes for the melting of aluminium scrap. The solubility of CaF_2 in the chloride salt system is limited. Dissolved CaF_2 is considered to improve the properties of salt fluxes. Experimental results indicated that the solubility of CaF_2 in 70 wt percent NaCl/30 wt percent KCl, German salt flux used in aluminium scrap melting, might be lower than published previously. As a consequence the pseudobinary section CaF2-(70 wt percent NaCl - 30 wt percent KCl) was investigated in detail by differential thermal analysis (DTA). The determined solubility of CaF_2 appeared to be considerably lower than presented in the literature and is 2.6 percent at 750 deg C. The pseudo binary section calculated using thermodynamic data confirmed the results to a large extent. DTA allowed also determining the crystallization temperature interval of the two phase area consisting of the solid solution NaCl-KCl and CaF_2. Based on these results semi production scrap melting tests in a salt bath were conducted. A calculating model for metal loss in the salt in dependency of CaF2-content, temperature and salt/scrap ratio was developed. The F-loss during the scrap melting was quantified. The experimental data confirms the theoretical recommendation for using a 30:70 KCl-NaCl-mixture with 2-3 percent CaF_2 and low salt/scrap ratios.