Patent Number: 047553286
Section: description

GENERAL DESCRIPTION OF THE INVENTION According to the invention there is provided a process for treating, decontaminating and adjusting the pH of acid uraniferous solutions, said process comprising treating the solutions, having an initial pH of about 2.5 to about 6.5 or of which the pH is previously adjusted within the range of about 2.5 to about 6.5 and containing about 1 to 100 mg/l of uranium, adding an aluminium salt soluble in the solutions, and which salt, after hydrolysis in the solutions, results in the formation of Al(OH).sub.3 and is liable to account for an increase in the pH, the addition of this aluminum salt being effected in a sufficient amount so that the final pH is from about 5.5 to about 3.5, and so that there is precipitation, coagulation and adsorption of at least 90% of the uranium initially contained in the solution, and so that the content of uranium remaining in the final solution obtained, is equal to or less than 1.8 mg/l. The acid uraniferous solutions treated according to the invention are either drained waters, or come from the acid lixiviation treatement of uranium ores. The pH of the acid solutions, treated according to the process of the invention, is generally comprised from about 2.5 to about 5.5. The uraniferous solutions treated according to the invention may also be liquors of initial pH of about 6.5 to about 8, previously acidified to pH of about 2.5 to about 6.5, particularly to about 2.5 to about 5.5, by the addition of a suitable amount of an acid. The process according to the invention is advantageously applied to solutions whose initial pH is from about 6.5 to about 8, containing at least about 1 g/l of sulfate ions, and whose pH is previously brought to the value of about 2.5 to about 6.5, particularly from about 2.5 to about 5.5, by the addition of a suitable amount of acid, particularly sulfuric acid. The uranium present in the acid uraniferous solutions, treated by the process of the invention is either in soluble form and/or in colloidal form. In the acid solutions treated according to the process of the invention, the solubilised form and the colloidal form of the uranium generally coexist in respective proportions which depend on the pH and the nature of the ions in solution. To fix ideas, it may be considered that within the pH range from about 2 to about 6, the uranium is to a large extent solubilised, particularly in the form of uranyl sulfate UO.sub.2 (SO.sub.4).sub.3.sup.4-, but it also exists in colloidal form. On the other hand, it may be considered, within the pH range from about 6 to about 7.5, a fortiori from about 6 to about 6.5, that the uranium is essentially in colloidal form, which does not exclude the presence of uranium in solubilised form. The aluminum salt used soluble in aqueous medium, particularly in the solutions to be treated, is hydrolysed after having been added to the solutions to be treated and there is formation of aluminum hydroxide Al(OH).sub.3, which can coagulate and adsorb the colloidal uranium present in the solution to be treated. In other words, the aluminum salt plays the role of coagulant with respect to the colloidal uranium. It is recalled that the colloidal form corresponds to a phase constituted by particles so small that the forces at the surface play on important part in its properties. The sizes of the colloidal particles are from 10.sup.-1 to 10.sup.-3 microns. They are constituted by associations of molecules or by small crystals charged as a result of the adsorption of ions and thus separated from the solution by a double layer. It is also recalled that the coagulant permits the separation of a colloidal suspension. This separation from the suspension necessitates recourse to artificial means. This operation is summarised by two different actions: destabilisation by addition of chemical reagents which, by mechanisms of agregation or adsorption, cancel the repellent forces or act on the hydrophilic nature of the colloidal particles; agglomeration of the "discharged" colloids: it results from various forces of attraction between particles placed in contact, first by Brownian movement until the obtention of a size of about 0.1 micron, then by external mechanical stirring bringing the flocks to a sufficient size. The coagulating action of the aluminum salts used in the invention result from the hydrolysis which follows their dissolution, without leading immediately to the formation of aluminum hydroxide. The intermediate compounds of aluminum, hydroxo-aluminous complexes, bring charges which are necessary for the neutralisation of the colloids hence creating bridges between the colloids and initiating the floculation process. It should also be noted that pH plays every important play in the study of coagulation-flocculation phenomena. In addition, the aluminum salt added to the solution to be treated is such that the aluminum form part of the anion, and the cation of this salt, after the hydrolysis of the abovesaid salt in the solution to be treated, can result in an increase in pH, which causes the precipitation of the uranium, particularly in the form of uranyl hydroxide. The amount of aluminum salt to be added is such that, on the one hand, coagulant is formed sufficiently in the uraniferous solution to be treated, to coagulate and adsorb the colloidal uranium and such that, on the other hand, the pH is taken to a value from about 5.5 to about 8.5, a suitable value for the precipitation of the solubilised uranium. In a preferred embodiment of the invention, the amount of aluminum salt added must be such that the final pH is from about 6 to about 7.5, since this pH range corresponds to the solubility minimum of the Al.sup.3+ ions of the coagulant used and enables the coagulation and adsorption of the maximum of colloidal uranium contained in the solution to be treated. If too much aluminum salt is added, the pH increases and exceeds the upper limiting value corresponding to the solubilisation minimum of the Al.sup.3+ ions; the Al.sup.3+ ions are then again found in the solution in stronger or weaker amounts according to the mineralisation of the solution, and the uranium is redissolved. The use of the process of elimination of uranium according to the invention can allow the elimination of the totality of the uranium, but the elimination of at least about 90% is sufficient to obtain uranium contents below about 1.8 mg/l. The examples indicated below, show that in practice, from about 95 to about 98% of the uranium initially present is eliminated. Among the aluminum salts used in the process according to the invention, recourse is advantageously had to an aluminate of an alkali or alkaline earth metal or to ammonium aluminate. It is also possible to contemplate the use of a mixture of aluminum salts. In a prefered embodiment of the invention, sodium aluminate is used. In aqueous medium, sodium aluminate behaves as indicated by the following reaction: EQU AlO.sub.2 Na+2H.sub.2 O.fwdarw.NaOH+Al(OH).sub.3 The sodium aluminate used is available commercially and is found in solution at the concentration of about 1,400 g/l of AlO Na and containing about 16% of Al.sub.2 O.sub.3 and about 20% of Na.sub.2 O. It is also possible to use sodium aluminate whose concentration is about 1,500 g/l of AlO Na and containing about 23% of Al.sub.2 O.sub.3 and about 18% of Na.sub.2 O. Instead of the aluminum salt, it is also possible to contemplate to add Al(OH).sub.3 directly, but this hydroxide being poorly soluble, it is more advantageous to resort to the preparation in situ of Al(OH).sub.3, by the addition to the solution to be treated of a soluble salt, since aluminum hydroxide thus freshly prepared is more active and does not account for a solubility problem. In a preferred embodiment of the process according to the invention, when the initial pH of the solution to be treated is less than about 5, in a first step the pH of the solution is raised to the value of about 5, by the addition of a base, then in a second step, the aluminum salt is added to the solution to be treated. By increasing the initial pH of the solution to be treated by the addition of the base, it is thus possible to remove about 60% of the uranium present in the initial solution; then an aluminum salt is added in suitable amount to obtain a solution whose pH is comprised from about 5.5 to about 8.5, particularly from about 6 to about 7.5, and to precipitate and coagulate the uranium remaining in the solution in solubilised and/or colloidal form; the coagulated and/or precipitated uranium is then removed and the final solution obtained contains uranium in an amount less than or equal to 1.8 mg/l. The combination of these two steps has the advantage of reducing the amount of aluminum salt to be added and improving the removal of the uranium initially present in the solution to be treated. As a base, soda is advantageously used, for example, at a concentration of about 300 to about 400 g/l, in a proportion of about 300 mg/l of solution to be treated. According to another preferred embodiment of the process of the invention, about 10 to about 250 mg of aluminum salt per liter of solution to be treated is generally used. The amount of aluminum to be added varies not only according to the amount of uranium to be removed but also according to the mineralisation of the solutions to be treated. By mineralisation, is meant the presence in larger or smaller amounts of calcium, magnesium, sodium, sulfate, ferric, chloride, carbonate, bicarbonate, phosphate, potassium, nitrate, silicon, aluminum ions initially present in the solution. Typical solutions of the invention contain: from about 0 to about 6,000 mg/l of SO.sub.4.sup.-- ions PA1 from about 0 to about 1,000 mg/l of CO.sub.3.sup.-- ions PA1 from about 0 to about 2,000 mg/l of HCO.sub.3.sup.- ions PA1 from about 0 to about 600 mg/l of Ca.sup.++ ions PA1 from about 0 to about 200 mg/l of Mg++ ions PA1 from about 0 to about 3,000 mg/l of Na.sup.+ ions PA1 from about 0 to about 4,000 of Cl.sup.- ions PA1 from about 0 to about 100 mg/l of K.sup.+ ions PA1 from about 0 to about 10 mg/l of NO.sub.3.sup.- ions PA1 from about 0 to about 60 mg/l of silicon ions with reference to SiO.sub.2 PA1 from about 0 to about 10 mg/l of Al.sup.3+ ions PA1 from about 0 to about 5 mg/l of Fe.sup.3+ ions PA1 from about 0 to about 1 mg/l of PO.sub.4.sup.3+ ions PA1 from about 100 to about 600 mg/l of Ca.sup.++ PA1 from about 100 about 200 mg/l of Mg.sup.++ PA1 from about 200 to about 3,000 mg/l of Na.sup.+ PA1 from about 500 to about 6,000 mg/l of SO.sub.4.sup.-- PA1 from about 100 to about 4,000 mg/l of CL.sup.- PA1 about 60 mg/l of Ca.sup.++ PA1 about 60 mg/l of Mg.sup.++ PA1 about 150 mg/l of Na.sup.+, particularly about 25 mg/l of Na.sup.+. PA1 about 250 mg/l of SO.sub.4.sup.-- PA1 In the case of a weakly mineralised solution, the aluminum salt is added in the proportion of about 10 to about 100 mg/l of solution to be treated. PA1 2 mg/l of U PA1 116 pCi/l of Ra PA1 811 mg/l of SO.sub.4.sup.-- PA1 470 mg/l of Ca.sup.++ PA1 &lt;5 mg/l of Al.sup.3+ PA1 &lt;1 mg/l of Fe.sup.3+ PA1 40 mg/l of Mg.sup.++ PA1 25 mg/l of Na.sup.+ PA1 24 mg/l of Cl.sup.- PA1 traces of CO.sub.3.sup.- PA1 44 mg/l of HCO.sub.3.sup.- PA1 &lt;0.1 mg/l of PO.sub.4.sup.+++ PA1 29 mg/l of SiO.sub.2 PA1 18 mg/l of K.sup.+ PA1 8 mg/l of NO.sub.3.sup.- PA1 7.5 mg/l of U PA1 775 mg/l of SO.sub.4.sup.-- PA1 109 mg/l of Fe.sup.3+ PA1 29 mg/l of Al.sup.3+is treated. By "highly mineralised" solutions is meant below, solutions in which the total concentration of ions is higher than 1 g/l. Typical "highly mineralised" solutions treated by the process according to the invention contain for example: In the case of a highly mineralised solution, the aluminum salt is added in the proportion of about 50 to about 200 mg/l of solution to be treated. By "weakly mineralised" solutions is meant below, solutions in which the total concentration of ions is less than 1g/l, particularly less than 0.5 g/l. Typical "weakly mineralised" solution treated by the process according to the invention contain less than: According to a preferred embodiment of the process of the invention, after the precipitation, the adsorption and the coagulation of the uranium, the solid particles of uranium thus formed are removed from the solutions, particularly by decantation. A preferred embodiment of the process according to the invention comprises an additional step, whose purpose is the elimination of the radium, which may also be contained in the uraniferous solutions to be treated. After the process as defined above for removing uranium, from acid uraniferous solutions, has been set up at the end of which the uraniferous solutions obtained can contain less than about 1.8 mg/l of uranium and have a pH comprised from about 5.5 to about 8.5, particularly from about 6 to about 7.5, the radium is removed by precipitating it in the form of radium sulfate by the addition, in the presence of sulfate ions, of barium chloride in sufficient amount for the content of radium ions remaining in the solution to correspond to an activity equal to or less than about 10 pCi/I. The operation of eliminating the radium is done under conditions such that there are no substantial changes in the value of the pH of the solution obtained at the end of the elimination step of the uranium. In the course of the precipitation of the radium sulfate, the barium sulfate, still present, also co-precipitates. After this step, a separatation between the solid particules formed of radium and the solutions is carried out, particularly by decantation, which permits solutions to be obtained containing a concentration of uranium equal to or less than 1.8 mg/l and of radium such that it corresponds to an activity equal to or less than 10 pCi/l. To simplify the expression "concentration of radium corresponding to an activity expressed in picocurie per liter (pCi/l)", in the rest of the description, the expression "concentration of radium in picocurie per liter (pCi/l)" will be used. For example, the expression "concentration of radium of 10 pCi/l" means "concentration of radium corresponding to an activity of 10 pCi/l". It is also possible to proceed with the treatment of the elimination of the radium on solutions which have undergone the process of elimination of the uranium, as indicated above, but in which the soluble uranium particles formed have not been removed from the solutions. At the end of the precipitation of the radium, separation is then effected of the solid uranium and radium particles, particularly by decantation, which permits solutions to be obtained containing a concentration of uranium less than or equal to 1.8 mg/l and a concentration of radium equal to or less than 10 pCi/l. The uraniferous solutions to be treated contain generally from about 10 to about 2,000 pCi/l of radium. The barium chloride used is available commercially and is delivered in the form of solutions containing about 350 g/l of barium chloride. When barium chloride containing about 350 g/l of BaCl is used, the amount of barium to be added generally varies from about 10 to about 20 mg/l, according to the solutions to be processed. The content of the uraniferous solutions advantageously treated by the process according to the invention is such that the sulfate ions are in sufficient amount for the precipitation of the radium to be almost complete and for the final solution obtained to contain less tha 10 pCi/l of radium. It is interesting to note that taking into account the amounts of barium chloride generally used to treat the solutions of the invention, the content of chloride ions introduced by the barium chloride is very low, in the vicinity of about 5 mg/l, which content in general is very much less than the amount of chloride ions contained initially in the solutions to be treated. According to another embodiment of the process of the invention, the steps constituted by the removal of the uranium and the removal of the radium may be reversed. DESCRIPTION OF PREFERRED EMBODIMENTS The following examples, given purely by way of indication, will permit a better understanding of the invention, but are not to be taken as in any way limiting the scope of the invention. EXAMPLE 1 The process according to the invention is used to treat an acid uraniferous solution (drained waters) of initial pH 5.28, and containing: To remove the uranium, sodium aluminate is added, in the proportion of about 100 mg/l of solution to be treated. The sodium aluminate used is marketed by the RHONE POULENC company. It is delivered in the form of a solution of about 1,400 g/l of AlO.sub.2 Na, containing about 16% of Al.sub.2 O.sub.3 and about 20% of Na.sub.2 O. To remove the radium, barium chloride is used, in a proportion of about 10 mg/l of solution to be treated. The barium chloride used is marketed by the Rhone Poulenc company. It is delivered in the form of a solution containing about 350 g/l of BaCl. After treatment, the final pH is 6.92, the concentration of radium is 2 pCi/l and the concentration of uranium is 0.3 mg/l. EXAMPLE 2 By the process according to the invention, an acid uraniferous solution (drained waters), of initial pH 2.77 containing no radium and containing: In a first step, soda is added, whose concentration is about 300 g/l, in the proportion of about 300 mg/l of solution to be treated. The pH of the solution so obtained is about 5. This increase in pH leads to the precipitation of about 60% of the amount of uranium initially present, which can then be removed. In a second step, sodium aluminate, having the same characteristics as in Example 1 is introduced, in the proportion of 200 mg/l of solution to be treated to remove the uranium still present in the solution. The final solution obtained has a pH of 6.3 and uranium content below 0.1 mg/l. EXAMPLE 3 In accordance with the process of the invention, the treatment is applied to a solution of initial pH 2.87 (drained waters), containing no radium, and containing 8.7 mg/l of uranium. The respective contents of the SO.sub.4.sup.--, Fe.sup.3+ and Al.sup.3+ ions are in the vicinity of those indicated in Example 2. In a first step, soda is added, whose concentration is about 300 g/l, in the proportion of about 300 mg/l to obtain a pH of about 5, which leads to the precipitation from the solution to be treated of about 60% of the uranium initially present. In a second step, sodium aluminate is introduced having the same characteristics as in Example 2, in a proportion of about 100 mg/l of solution to be treated, to remove the uranium which is still present in the solution. The final pH of the solution is 6.8 and the concentration of the uranium is less than 0.1 mg/l.