Oxide & Solid Formation
After the completion of SX-based REE separation and purification, what is left is a strip solution containing, in dissolved form, the desired REE or mix of REEs at the desired purity and specification. From this solution, which is usually strong HCl, but can also be strong nitric acid, the REE can be removed through compound formation or precipitation. A few examples are given below:
Oxalic acid can be added to the strip solution, and will quantitatively form REE oxalate precipitate. This product is very convenient to process as the REE oxalate is very granular in form, and easy to filter. After the REE oxalate is filtered and washed, it is further processed to the oxide with the use of either a rotary kiln for a continuous process or a muffle furnace for a batch process. During this stage, the oxalate transforms into the oxide. Next it is cooled, sampled for assay, and packaged.
Note, freshly-fired oxide is hydroscopic, especially lanthanum and neodymium oxide, and rapidly absorbs moisture from the air. Due to this effect, the volume of the material expands and can breach the container if packed too tightly. Also, note that producing from chloride media will inevitably leave some chloride in the product, possibly causing concern to the customer. Precipitation from nitric media will also leave residual nitric, but this is volatilized and removed during the firing process, whereas much of the chloride remains after firing.
Neodymium Oxide (Nd2O3)
Many REE products can be formed, depending upon customer need; fluorides, carbonates, acetates, etc. are all produced in a similar manner. For example, to produce the fluoride, a precipitating F-containing compound is added to the strip solution, pH adjusted as required, and the REEF3 is formed. It is then filtered, washed, dried or calcined, and packaged.
When attempting to produce very high purity compounds, special focus needs to be on the materials used in the precipitation stage, as unwanted impurities can be accidentally added. The purity of the precipitating compound may not be as high as the REE strip solution, and needs to be assayed. The wash water can also add various impurities and needs to be especially clean. Impurities can even be transferred from the furnace to the product, or from the containers used in the furnace, ambient dust, residual processing materials, or even the furnace liner itself.