الفهرس 
General StatementOnly 14 pages are availabe for public view
 |  | from | 167 |  |  |
| | | from | 167 | | |
Abstract
The presence of harmful elements in uranium concentrate is a serious problem during nuclear fuel fabrication and performance. The presence of B and Cd in a fraction of ppm results in neutrons capturing while the presence of Fe and Co causes a hardening of the cold rolled U metal. Also presence of Nb, Ta, and Mo tend to form volatile fluorides which trouble the enrichment of U235in the diffusion plant. The target is removal of such harmful elements present in dry crude yellow cake (sodium diuranate) concentrate produced from Gattar Pilot Plant i.e. purification process. This can be achieved by adsorption of some harmful elements onto synthetic polymers and optimize the relevant factors in a manner to obtain a purified uranium concentrate that is would be suitable for next steps as a nuclear fuel fabrication and meet the permissible limits of the nuclear standard specification.
To realize the objectives of this work, it was found necessary to give a brief summary about the two topics that would be covered in the present work. These include uranium ore processing and the synthetic polymers which is actually considered as a representative of the new trend of applying solid phase extractants for metal ions removal from its solutions.
There are two available processes for uranium leaching from its mineralization, acid leaching and alkaline leaching. Sulfuric acid and sodium carbonate are generally used as the lixiviants depending on ore materials. Acid leaching is generally preferred, because it gives higher results for uranium dissolution, unless the ore contains minerals that would render the acid consumption prohibitively excessive like carbonate materials.
Alkaline leaching is generally preferred in case of ores that contain more than 4% carbon dioxide on the condition that the sulfide content does not exceed 0.5% sulfur. The alkaline leaching would require fine grinding with subsequent filtration problems.
Classical methods have generally been applied to meet the required specification in the final product (yellow cake); namely, the solvent extraction technique and the ion exchange procedure. The former involves several types of organic solvents. On the other hand, the developed ion exchange resins belong essentially to the anionic types; a matter which depends on uranium existence in the form of anionic sulfates or carbonates complexes.
The solid phase extractants (SPE) have been introduced more recently for purification of metal ions. The advantages of SPE are actually reduced energy consumption as well as their disposal costs and extraction time. Chitosan (CS) has been chosen in the present work for metal ions removal from the nitrate solution of the dry crude yellow cake produced at G. Gattar experimental pilot plant. Cross linking reaction of chitosan with epichlorohydrin reinforces its solubility in acidic media and the produced CCS has reactive amino and hydroxyl groups. For the preparation of CCS, chitosan (CS) was dissolved in acetic acid followed by the gradual addition of ECH followed by DROP wise addition of 5% (w/v) NaOH solution where the ECH-CCS was thus obtained as a white solid precipitate. The reaction of CS with ECH was through cross-linking at the hydroxyl groups. For the preparation of PAN by polymerization, most part of water and acrylonitrile (AN) were charged into the flask and stirred intensively. When the temperature was reached at 60º C, KPS dissolved in the rest part of water was injected. Synthetic polymers are used in addition to polyvinyl alcohol which contains high polar hydroxyl groups in removal of some harmful elements from nitrate solution of dry crude yellow cake.
The prepared dry crude Y.C. usually contains about 70–85% of uranium as U3O8 beside some toxic impurities which have harmful effects on nuclear fuel fabrication and performance. Accordingly, refining step is one of the most important steps in nuclear fuel cycle that minimizes these harmful impurities to the acceptable levels in agreement with the international standard specification of nuclear purity. This can be achieved via adsorption of harmful elements upon the synthesized epichlorohydrin cross-linked chitosan (CCS), polyacrylonitrile (PAN) and polyvinyl alcohol (PVA).
For this purpose, a nitrate solution was prepared by dissolving a proper weight of dry crude yellowcake in dilute nitric acid where a nitrate solution assaying 400 ppm U was obtained. characterization of the synthesized polymers in the present work namely; the FT-IR, average molecular weight and the ESEM-EDAX were performed.
To optimize the relevant adsorption factors, several series of batch adsorption experiments have been performed using 10 mg of the synthesized CCS, 10 mg of the synthesized PAN, 50 mg of PVA and 30 ml of the prepared Gattar yellow cake nitrate solution for each one. These factors included the contact time, the pH, the temperature, the adsorbent dosage as well as the initial metal ions concentration. from the obtained results, it was concluded that the optimum conditions for metal ions removal upon the synthesized CCS, PVA and the synthesized PAN using 30 ml of the prepared uranyl nitrate solution (400 mg / L) can be summarized as follows:
• A contact time of 180 min.,
• A pH value of 6
Under these conditions, it was possible to obtain about 77.80%, 89.9%, 35.97%, 87.5%, 78.12%, 100 % and 93% removal efficiency for Ba (II), V (II), Ti (II), Mn (II), Ni (II), Cu (II) and Zn (II) on CCS. On the other hand, the removal efficiency of uranium was found to decrease to 25 % at pH 6.
In case of using PVA, it was possible to obtain about 62.5%, 44.90 %, 87%, 46.2%, 100 % and 82.6% removal efficiency for Ba (II), Ti (II), Mn (II), Ni (II), Cu (II) and Zn (II) on PVA. On the other hand, the uranium removal efficiency was found to decrease to 25 % at pH 6.
In case of using PAN, it was possible to obtain about 87%, 80 %, 25 % and 46% removal efficiency for Mn (II), Ni (II), Cu (II) and Zn (II) on PAN. On the other hand, the uranium removal efficiency was found to decrease to 10 % at pH 6.
Concerning the physical parameters of the working system, it was found necessary to calculate the parameters of both the equilibrium isotherm as well as the adsorption kinetics. For the former, the Langmuir equilibrium isotherms have been calculated and it was indicated that Mn (II), Ni (II) and Zn (II) adsorption on the CCS and PAN correlated well with the Langmuir equation (R2 ≈0.9) with a total maximum adsorption capacity of 772 and 551mg /g, respectively. Thus, a monomolecular layer of metal ions formed on the adsorbent at saturation point.
On the other hand, the adsorption of Mn (II), Ni (II) and Zn (II) onto PVA not correlated well with the Langmuir equation. It found that the Freundlich isotherm model gave the highest R2 value, and fitted the adsorption data better than Langmuir model isotherm and the total maximum adsorption capacity of Mn (II), Ni (II) and Zn (II) onto PVA was 171 mg /g. By comparison the total Qmax of CCS, PAN and PVA, it was found that the total Qmax of PVA is lower than the total Qmax of CCS and PAN.
On the other hand, from the parameters of the adsorption kinetics calculated from the obtained experimental data; it was found that based on the correlation coefficients, the adsorption of Ni (II), Mn (II) and Zn (II) on CCS, PVA and PAN are well described by both the pseudo-first-order and the pseudo-second-order equation, except the adsorption of Mn (II) on PVA is described by the pseudo-second-order equation, indicate to the adsorption processes on CCS, PVA and PAN are mixed adsorption (chemo and physisorption). The intraparticle diffusion model linear plots for CCS, PVA and PAN indicated that the intercepts C were not zero, so the intraparticle diffusion may not be the factor in determining the kinetics of the adsorption process.
Also, it has to be indicated that, the negative value of enthalpy change ∆Hoads for the three metal ions on CCS, PVA and PAN further confirms the exothermic nature of the adsorption process as room temperature is the best for adsorption while the entropy of adsorption ∆So ads negative values reflects metal ions were orderly adsorbed onto the adsorbent surface. Also, the negative free energy values ∆Goads indicate the feasibility of the process and its spontaneous nature. On the other hand, the positive ∆Ho ads value of for Ni+2 at PVA reflects endothermic nature of the adsorption processes and the positive entropy ∆So ads value for Ni(II) at PVA and Zn(II) at PAN reflects the affinity of the polymers toward Ni(II) and Zn(II).