Effective recovery of rare earth and fluorine from REF3 smelting slag by a mechanochemical process

[Display omitted] •Rare earth fluoride completely being converted into hydroxide rare earth at room temperature via mechanochemical process.•High recovery rate of fluorine in the rare earth fluoride.•Mechanical force destroy the dense product layer on the surface of rare earth fluoride to promote the conversion of rare earth fluoride.•Green, environment-friendly, efficient, zero emission, and simple process. The rare earth fluoride (REF3) solid wastes (such as REF3 smelting slag, rare earth molten salt electrolytic slag, and cerium fluorocarbon tailings, etc.) are widely sourced and have high recycling value. Traditional technology for the recovering of rare earths from REF3 solid wastes mainly includes the method of concentrated sulfuric acid roasting, high-temperature concentrated alkali boiling, and alkali roasting, but sulfuric acid roasting will produce harmful gases containing fluorine and sulfur, which seriously pollute the environment; high temperature concentrated alkali boiling and alkali roasting method also has the disadvantages of large consumption of chemicals and heavy burden of wastewater treatment. In this work, a mechanochemical process was introduced to treat the REF3 smelting slag (RSS) produced by calcium-thermal reduction of REF3, and then the product was leached directly by dilute acid. The results showed that REF3 can be completely converted to rare earth hydrate (RE(OH)3) by the mechanochemical process, and the REEs in the product can be leached efficiently by HCl at room temperature. In particular, the properties of the mechanochemical process and the acid-leaching process were studied. Under the optimal operation conditions (sodium hydroxide/RSS mass ratio was 0.4:1, milling speed was 400 rpm, milling time was 40 min), the transformation rate of REF3 was above 96%. The residue consisted of RE(OH)3 and CaF2 were obtained by leaching the product with water. The NaF crystal by-product was obtained by evaporation crystallization of filtrate. The REEs leaching rate was above 96% by acid-leaching the residue under the optimal acid-leaching process (2 mol/L hydrochloric acid, liquid–solid ratio was 5:1, acid-leaching time was 20 min, stirring speed was 600 rpm), and the by-product of CaF2 was obtained after filtration. The whole process was carried out at room temperature, in a short time, with zero-emission, and recyclable water, which is in line with the concept of green, clean, and environment-friendly, and is expected to be a new te