Decomposition of monazite concentrate in potassium hydroxide solution

Abstract

Alkali roasting of monazite concentrate using potassium hydroxide in place of sodium hydroxide could allow for the coproduction of tripotassium phosphate, which would be of greater economic value than trisodium phosphate. Therefore, this study aims to assess the ability of potassium hydroxide to extract rare earth elements (REEs), uranium (U) and thorium (Th) from monazite concentrate under similar conditions to those that have been established using sodium hydroxide. A process consisting of alkali pug roast followed by a nitric acid leach has been examined with experimental parameters including roasting temperature (150–250 °C), roasting time (1–4 h), and alkali to concentrate (A/C) weight ratio (1:1–4:1). Overall, potassium hydroxide was found to behave similarly to sodium hydroxide under the conditions studied. The use of potassium hydroxide also helped alleviate the problem of overroasting that occurs with sodium hydroxide. Under relatively high temperatures and extended roasting times, REEs extraction decreased less when compared to sodium hydroxide. Oxalic acid was then used to precipitate REEs, U and Th from the nitric acid leach solution. Oxalic acid dosage was examined to determine the minimum amount needed to effectively precipitate the REEs at room temperature. Precipitation time and temperature were also studied for their effects on the overall precipitation. It was found that 6.2 %w/v oxalic acid was able to precipitate more than 90% of yttrium (Y), lanthanum (La), cerium (Ce), neodymium (Nd), and praseodymium (Pr). Thorium precipitation remained nearly complete (>95%) under all oxalic acid concentrations studied.