3.1. Studying the Distribution of Radioactive Impurities during Autoclave Leaching and Subsequent Processing
It is known that in the averaged composition of the Krasnoufimsk monazite sample, there is not only up to 67% of the sum of high-priced rare-earth oxides, but also 6.9% of thorium oxide ThO
2 and 0.18% of uranium oxides (
Table 1). Thus, the initial mineral raw material is a radioactive material containing alpha emitters (
232Th,
235U,
238U) and their decay products (
228Th,
224Ra,
226Ra,
212Bi,
220Rn,
222Rn, and
212,214,216,218Po), some of which are gamma-active (
228Ac,
212,214Bi,
208Tl,
212Pb). The chain of radioactive transformations of
232Th is shown in
Figure 1 diagram below:
Therefore, before performing the technical operations of opening and extracting of REEs, uranium, and thorium from monazite concentrate, it is necessary to collect the necessary information on the distribution of radioactive impurities during processing and on the activity levels of liquid and solid phases.
In the course of the research, the following substances were subjected to radionuclide analysis: (1) precipitation of the obtained REE oxides/hydroxide compounds, elements of the uranium series and impurity elements; (2) a phosphate-alkaline solution; (3) a solution obtained by dissolving hydroxides in nitric acid; and also (4) the part of the solid phase that did not dissolve, which are formed in the technological process (
Figure 2) [
20].
It was found that, basically, all gamma and alpha-active radionuclides are concentrated in the deposits of REE oxides/hydroxides, elements of the uranium series, and impurity elements obtained as a result of alkaline treatment of monazite concentrate (
Table 2 and
Table 3).
The analysis of alpha and gamma spectra showed that the main identifiable radionuclides in the samples were 228Ac, 212,214Bi, 212,214Pb, 208Tl, 228,232,234Th, 212Po, 224Ra, and 234Pa.
On the contrary, in all experiments, the phosphate-alkaline solution obtained after the autoclave opening of the concentrate did not contain any radioactive impurities, as evidenced by the results of both gamma and alpha spectrometric measurements. In this regard, it can be concluded that the phosphate-alkaline (clarified) solutions formed in the processing, as well as the wash water, are free from radionuclides of uranium, thorium, and radium; they do not require special treatment and can be used to obtain a by-product, phosphate fertilizers.
It was shown that the solution obtained by dissolving hydroxides and hydrated oxides of REEs, uranium, thorium hydroxides in nitric acid contained impurities of radium-224 (
Table 2 and
Table 3). However, due to its short half-life (about 4 days), the purification of the nitric acid phase from radium-224 is not expedient. In the authors’ opinion, the sludge that was not dissolved in HNO
3 in terms of the content of radionuclides can be classified as industrial waste that does not require separate burial.
3.2. Opening of Monazite Concentrate
The heterogeneous high-temperature process of the hydroxide opening of monazite concentrate is carried out according to the general scheme:
In this regard, according to the above scheme and taking into account the optimal conditions recommended in the literature [
28] for the complete conversion of monazite concentrate into the form of hydroxides and hydrated metal oxides in an autoclaved alkaline environment, it was necessary to determine: (1) the optimal conditions for the quantitative washing of the resulting hydroxide precipitates from Na
3PO
4 and NaOH after autoclave processing; (2) the optimal conditions for the dissolution of hydroxide pulp in nitric acid; (3) approaches to the selective isolation and separation of target components: REEs, uranium, and thorium from the substance purified from natural radionuclides by the extraction method.
In the first stage, the conditions for washing hydroxide precipitates obtained after the autoclave opening of monazite concentrate from the remainders of NaOH and phosphate ions, primarily Na3PO4, were studied. During the tests, it was experimentally observed that the quantitative washing of 10 g of the product of opening the monazite concentrate from both NaOH and Na3PO4 to the content of phosphate ions in the wash water < 10−3 g/L is achieved by a single wash with 100 mL of distilled water at 80 °C and contact time of 10 min. Trisodium phosphate in the environment of the alkali remaining after processing (up to 30%) at 100–110 °C easily crystallizes and is separated by a “blue ribbon” filter. In this case, Na3PO4 is isolated separately as a valuable by-product as a mineral fertilizer, and residual sodium hydroxide can be reused in experiments.
The second stage of research was focused on determining the optimal conditions for the dissolution of the obtained precipitation of hydroxides and hydrated oxides of REEs, uranium, and thorium in nitric acid after the autoclave processing of the concentrate.
To this purpose, the samples of monazite concentrate were autoclaved with alkali according to the procedures described in the previous section.
In the X-ray diffraction patterns of alkaline autoclave products there are no peaks assigned to the initial phases in the monazite concentrate, which indirectly confirms the completion of its opening (
Figure 2).
The washed and dried samples of hydroxides and hydrated oxides of REEs, uranium, and thorium were dissolved in HNO
3 depending on acid concentration, temperature, Solid/Liquid phase ratios (S/L), and their contact times. The resulting solubility is shown in
Table 4.
It appears that the main factors affecting the completeness of the dissolution of hydroxides and hydroxide precipitates of the concentrate components were the contact time and the concentration of HNO3. The S/L ratio and temperature values also affect the completeness of the dissolution of precipitation, but to a lesser extent. Under optimal conditions, the degree of opening of monazite concentrate reached values of up to 90–95%. On the contrary, a decrease in the contact time, S/L, and temperature, as well as an increase in the degree of fineness of the substance, reduces the degree of alkaline opening by 20–40%.
The introduction of an activating reagent H2O2 in the range from 0.01 to 0.1 mol/L does not significantly affect the degree of opening of the concentrate.
Based on the conducted research, optimal conditions for opening precipitates of rare-earth hydroxides, uranium, and thorium in HNO3 are proposed: for 1 g of substance, 50 mL of 9 mol/L HNO3, and heating for 3 h at 90 °C without adding hydrogen peroxide.
In the third stage, two approaches were proposed for the extraction of uranium, thorium, and REEs from nitric acid solutions during the processing of monazite concentrate.
Quantitative extraction separation of REEs, uranium, and thorium with the simultaneous purification of impurity chemical elements is a non-trivial problem. The main known technologies for opening monazite raw materials were focused on the complete separation of the target rare-metal products, while the separation of fractions of elements of the uranium series and thorium was a necessary stage guaranteeing radioactive purification from REEs [
30].
In the present study, two approaches were considered that were directly oriented to the extraction of thorium with a high yield of extraction from the mixture as a promising component of the thorium-uranium fuel cycle. The first approach was an attempt to move the components into two selective streams: a uranium stream and a stream of REEs and thorium. In the second approach, the separation of the components was carried out by co-separation of uranium and thorium into one selective stream followed by separation, and REEs into the second selective stream, followed by purification of rare-metal products by known methods at the second stage of the process.
The first approach included the extraction of uranium into a separate product, followed by purification from thorium-containing impurities by re-extraction with a complexing reagent. In the stage of uranium separation, 5% tributyl phosphate solutions were used as the extraction mixture, and 50% tributyl phosphate in S-13 at the stage of REE and thorium extraction. Data on the distribution of the components of the model solution are given in
Table 5.
It appears that when the proposed extraction scheme for separation and purification of REEs, uranium, and thorium is applied, and when the “5% tributyl phosphate—S-13” system is used, up to 90% U and up to 9.0% Th are extracted into the uranium branch. At the second stage of the process, the joint extraction of thorium and REEs into 50% tributyl phosphate in S-13, up to 80% with REE impurities in the organic phase was extracted into the selective thorium stream. In the aqueous phase, after the extraction of thorium, up to 100% of REEs remain in the raffinate. Thus, using the proposed scheme, it is possible to extract up to 90% of thorium from both streams.
The second approach is based on the joint quantitative separation of the sum of uranium and thorium into a separate branch, followed by a separation of the elements by re-extraction with diluted nitric acid under an elevated temperature (30–40 °C), which is traditionally used for the re-extraction of uranium in the Purex process [
31]. It was shown that the joint extraction of uranium and thorium into 30% tributyl phosphate in S-13 with their subsequent joint isolation for re-extraction operations allowed the extraction of 90.74% of thorium and 78.49% of uranium (
Table 6). The REE losses are not more than 4%.