Patent Publication Number: US-4321235-A

Title: Process for the treatment of alkaline liquors containing sulfate ions

Description:
The invention relates to a process for the treatment of an alkaline solution containing both sulfate ions and sodium ions for the sake of avoiding liquid effluents and recovering the sodium ions. 
     This process comprises contacting said solution with a cationic ion exchange resin in the ammonium form and, after the ion exchange has taken place, recovering the solution in which ammonium ions have been substituted for the sodium ions. 
     The sulfate ions are then precipitated in the form of an insoluble sulfate, particularly calcium sulfate. The sodium ions are recovered by elution from the resin. 
     The invention relates to a process for the treatment of an alkaline solution containing both sulfate ions and sodium ions, and more particularly sodium salts which are industrially useful. Particularly a solution of this type is constituted by the leach liquor obtained after attack or leaching of an uraniferous ore rich in sulfur compounds, particularly in the form of sulfides and/or sulfates, with a solution of sodium carbonate and/or bicarbonate in the presence of an oxidant, in order to extract uranium from the ore and to solubilize it in the liquor, particularly in the form of sodium uranyl-tricarbonate. Another solution of this type is formed by the liquid effluent obtained after separation of the uranium compounds from the above said liquor, such as by precipitation by adding sodium hydroxide to this liquor. 
     It is known that when the ores contain sulfur, particularly in the form of sulfides or sulfates, or both, alkaline leaching leads then to solutions rich in sodium sulfate. This content of sodium sulfate in the leach solutions represents a considerable drawback, because both of pollution and of expensive losses in reactants, such as sodium carbonate and/or bicarbonate. 
     In fact, it is well known that leach solutions based on sodium carbonate and/or bicarbonate are generally recycled to the leaching of other batches of ore, after the recovery therefrom of the solubilized uranium. This recycling is only possible however as long as the sulfate content does not exceed certain value. When the latter is reached the ore-treatment installations must be purged. When the ores have high contents of sulfur compounds, the losses of sodium are so high that uranium extraction processes by the alkaline route become uneconomical. 
     Additional sodium losses are further encountered upon having recourse to sodium hydroxide for precipitating the uranium contents from the uraniferous liquors, because only part of the added sodium hydroxide is then involved in the uranium precipitation. 
     In addition, the problem of the rejection of liquid effluents becomes all the more acute as the relative standards for environment protection tend to become more severe. Particularly liquid effluents containing high proportions of sulfates are often not permitted at all. 
     The problem of the treatment of effluents rich in sodium sulfate, under acceptable economic conditions, hence becomes pressing. The precipitation of sulfate ions in the state of insoluble barium sulfate, hence storable without serious pollution risks, cannot however be contemplated, by reason of the baryte or barium salts required for this treatment being too expensive. Precipitation of the sulfate ions, in the form of calcium sulfate, by treatment of the effluents with lime, is not satisfactory in practice because of the high stability of sodium sulfate. 
     It is an object of the invention to overcome these difficulties at least to a great extent, particularly to provide a treatment process applicable even to uraniferous liquors, obtained from uraniferous ores having high contents of sulfur compounds, whilst enabling at the same time both the suppression in practice of the rejection of highly polluting liquids and the recovery of the major part of the sodium unused in the course of the ore leaching and, possibly, of the precipitation of the uranium from the leach liquors with sodium hydroxide. 
     The process according to the invention therefore comprises 
     (a) contacting the starting solution with a cationic ion exchange resin in the ammonium form, and recovering the solution in which the salts and complexes previously of sodium, have been essentially converted into salts and complexes of ammonium, after the ion exchange has taken place; 
     (b) converting the ammonium sulfates contained in the solution obtained into insoluble sulfates, such as calcium 
     (c) eluting the sodium ions from the ion exchange resin with a solution of an ammonium salt, the anion of which is preferably that of the initial industrially useful sodium salts, such as a carbonate and/or bicarbonate. 
     In those cases where the initial solution would also contain solubilized uranium, the latter will then be separated from the solution obtained at the end of step (a), such as by precipitation by means of sodium hydroxide, prior to subjecting the solution then freed from its uranium contents to step (b) of the above defined process. 
     Advantageously the sulfate precipitation of step (b) is then achieved by adding calcium hydroxide to the solution. 
     Any strong cationic resin withstanding both carbonate-containing solutions, particularly at a pH between about 9.5 and about 10, and the corresponding alkaline solutions containing sodium hydroxide and other organic materials, if any, possibly extracted together with the uranium from the ore, can be used for running the process according to the invention. 
     Suitable resins are more particularly constituted by the sulfonic cationic resins. Advantageously, the polymer matrix of such strongly acid resins is formed of a styrene and divinylbenzene copolymer. They may be, either in the form of a gel without porosity, or in porous form, for example in a macro-crosslinked form with true pores of large sizes. Suitable resins, cited merely by way of example, are those marketed by the ROHM &amp; HASS company under the designations IR-120, IR-122 and IR-124. 
     If, as is customary, the cationic ion exchange resin is represented simply by the formula R--NH 4  when in the ammonium form, one may resort to the following chemical equations to understand the transformations that the principal sodium constituents contained in the liquor undergo in the course of the ion-exchange. 
     
         .sub.2 R--NH.sub.4 +Na.sub.2 SO.sub.4 →.sub.2 R--Na+(NH.sub.4).sub.2 SO.sub.4                                                  ( 1) 
    
     
         .sub.2 R--NH.sub.4 +Na.sub.2 CO.sub.3 →.sub.2 R--Na+(NH.sub.4).sub.2 CO.sub.3                                                  ( 2) 
    
     
         R--NH.sub.4 +NaOH→R--Na+NH.sub.4 OH                 (3) 
    
     When the treated liquor further contains uranium in the dissolved state, particularly in the form of sodium uranyl-tricarbonate, the latter undergoes the transformation which can be represented by the chemical equation below: 
     
         .sub.4 R--NH.sub.4 +Na.sub.4 UO.sub.2 (CO.sub.3).sub.3 →.sub.4 R--Na+(NH.sub.4).sub.4 UO.sub.2 (CO.sub.3).sub.3          ( 4) 
    
     Consequently it will be understood that most of the sodium initially contained in the solutions is retained on the ion exchange resin, from which it is then eluted upon recontacting the ion exchange resin with a solution of an ammonium salt. This recovery may be carried out directly in the form of sodium carbonate and/or bicarbonate, if the elution is effected with a solution of ammonium carbonate and/or bicarbonate. 
     The ammonium sulfate, which is much less stable than calcium sulfate, and the ammonium carbonate, and/or bicarbonate, if any still left, are easily precipitated, after separation of the possible uranium contained in the medium, by the addition of calcium hydroxide to the medium. 
     The ammonia released by this reaction constitutes, in a preferred embodiment of the process according to the invention, a particularly useful product for the production of ammonium carbonate which can then be used for regenerating the cationic resin, on which the sodium ions are retained, and consequently also for recovering that sodium, directly in the form of sodium carbonate and/or bicarbonate. A portion at least of these eluted solutions of sodium carbonate and/or bicarbonate may then, after concentration in an evaporator, be recycled to the leaching of new batches of uraniferous ore, under the usual conditions. Another portion of the eluted solutions of sodium carbonate can even, after making caustic by reaction with calcium hydroxide, be used for the production of a portion at least of the sodium hydroxide required for the uranium precipitation from the uraniferous extraction liquors, particularly when the process according to the invention is applied to uranium-free effluents. One can recover up to the water of the treated solution from the evaporator in which the concentration of the first mentioned portion of the eluted solution of sodium carbonate is effected. 
     When the process according to the invention is applied directly to an uraniferous extraction--or leach--liquor containing uranium, particularly in the form of ammonium uranyl-tricarbonate, various methods can be relied upon for recovering the uranium. 
     A particularly advantageous method for recovering the uranium from the solution obtained which contains the ammonium salts and/or complexes of uranium, consists of a heat treatment of the liquor, preferably at reflux temperature. The uranium is then precipitated in the form of a concentrated ammonium diuranate. This heat treatment produces also the decomposition of the ammonium carbonate into ammonia and carbon dioxide, which can be recovered for producing part of the ammonium carbonate necessary for producing the solution required for regenerating the cationic resin. 
     In a second type of preferred methods, the liquid containing the ammonium uranyl-tricarbonate is heated and acidified with sulfuric acid to a pH preferably of the order of 2.5 or even less, whereby the uranium compound is converted into soluble uranyl sulfate and the ammonium carbonate initially contained in the treated liquor is decomposed. The uranium sulfate can then, in a manner known per se, be precipitated by magnesia in the form of magnesium uranate. As in the preceding case, the carbon dioxide released as a result of the acidification of the medium, and the ammonia released during the conversion step of the uranyl sulfate into magnesium uranate, may be recycled to the production of ammonium carbonate, which may be used for producing a regenerating solution for the ion exchange resin. 
     It goes without saying that any other known process may be used for the separation of the uranium from either the still alkaline solution containing the uranium in the state of ammonium uranyl-tricarbonate, or from the acidified uranyl-sulfate solution. It is possible for example to resort to extraction processes employing organophosphorus solvents, such as di(ethylhexyl)phosphoric acid and to recover the uranium therefrom. 
     The residual liquor of which ammonium sulfate then remains the essential component, can then be treated with calcium hydroxide too, under the above described conditions, to precipitate the sulfate ions in the state of insoluble calcium sulfate. 
     The process of the invention is advantageous whatever the components still contained in the treated uraniferous alkaline liquors. 
     It enables the suppression of all liquid effluents. The final degraded products, essentially calcium sulfate and carbonate, are obtained in the solid state and are easily stored. 
     In addition the process according to the invention permits regeneration of the expensive materials and products used (ion exchange resin and sodium carbonate) and the transformation of the side products obtained (ammonium carbonate, ammonium hydroxide, ammonia and, in certain cases, even water) into valuable products. The only products which must be supplied in relatively large amounts in the course of the treatment cycle which has been described are constituted by particularly inexpensive industrial compounds: calcium hydroxide and carbon dioxide. 
     The process according to the invention can be applied successfully even to uraniferous leach liquors of ores containing both sulfur compounds and organic materials which, as is well known, are particularly difficult to process. In particular, it is applicable with advantage to ores which, such as those of the Herault area (France), have carbonates contents (essentially conventional dolomite or a ferruginous dolomite of the ankerite type), expressed as CO 2 , from about 5 to 10% by weight and contents of organic components, from about 1 to about 5% by weight, which components comprise constituents with a more or less marked graphitic character, hydrocarbons and various organic reducing agents, among which organic acids, denoted as &#34;humic acids&#34; and which, by alkaline attack and oxidation, are transformed into soluble &#34;humates&#34;. 
    
    
     Additional characteristics of the invention will appear also in the course of the description which follows of examples of the application of the process according to the invention. 
     EXAMPLE 1 
     Application of the process according to the invention to alkaline effluents obtained from an alkaline uranium extraction liquor from an ore, after the separation of the uranium 
     The composition of the treated effluent is the following: 
     
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Na.sub.2 SO.sub.4                                                         
             36.4           g/l                                           
Na.sub.2 CO.sub.3                                                         
             17.9           g/l                                           
NaOH         5.12           g/l                                           
U            9              mg/l                                          
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     The resin used was that marketed by the ROHM and HAAS company under the designation IR 120 , whose capacity in milli-equivalents (meq) was in the neighbourhood of 2. The operations were carried out with a column having a volume of 34 cm 3 , a height of 21 cm and a diameter of 1.5 cm. The effluent was percolated through the resin column, at flow rates and at temperatures indicated in the table below. 
     
         ______________________________________                                    
                      Amount                                              
                      of Na.sup.+  Amount of                              
              Temp.   retained                                            
                             Value NH.sub.4.sup.+  in                     
                                           Value                          
Tests                                                                     
     Flow     in      on resin,                                           
                             as    the liquor,                            
                                           as                             
Nr.  rate     °C.                                                  
                      in mg. meq   in mg.  meq.                           
______________________________________                                    
1    2.16 V/h 20      1.618  2.07  1.114   1.82                           
2    2.08 V/h 20      1.650  2.11  1.315   2.15                           
3    2.07 V/h 20      1.603  2.05  1.537   2.51                           
4    2.01 V/h 20      1.595  2.04   .983   1.61                           
5    2.46 V/h 25      1.708  2.18  1.215   1.98                           
6    1.44 V/h 40      1.626  2.08  1.183   1.94                           
7    2.37 V/h 40      1.483  1.79  1.169   1.91                           
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         ______________________________________                                    
Mean     1.603     2.05      1.218   1.99                                 
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         ______________________________________                                    
Standard                                                                  
devia-        0.12         0.283                                          
tion                                                                      
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     The results of this table show that an increase in temperature or a variation in the flow rate within the limits studied, do not modify the results obtained appreciably. The values in meq demonstrate that the exchange capacity of the resin is fully utilized by the process according to the invention. 
     By the addition to the solution of calcium hydroxide in the proportion of 100 to 400 grams per liter, the almost quantitative precipitation of the SO 4  and the CO 3  ions from the solution is produced in the state respectively of calcium sulfate and calcium carbonate. 
     The ions fixed on the resin were then eluted completely, as the table of results below shows. These results were obtained by causing a solution containing 70 to 150 grams of ammonium carbonate per liter to flow through the column. 
     
                       TABLE II                                                    
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Tests  Elution flow                                                       
                   Amount of                                              
Nr.    rate V/h    Na.sup.+  eluated                                      
                               milliequivalents                           
______________________________________                                    
1      1.05        1.673       2.14                                       
2      1.09        1.692       2.16                                       
3      2.58        1.728       2.21                                       
4      1.79        1.723       2.20                                       
5      2.01        1.722       2.20                                       
6      1.52        1.720       2.20                                       
7      2.03        1.747       2.33                                       
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         ______________________________________                                    
Mean         1.715         2.19                                           
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         ______________________________________                                    
Standard          0.031                                                   
deviation                                                                 
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     It is observed that the loading capacity of the column in ammonium ions was even shown to be slightly higher than the value stated by the manufacturer. Similarly, the flow rate, which corresponds to a flow rate of the solution which can reach 2.5 volumes of resin per hour, does not very much influence the regeneration capacity of the resin. 
     EXAMPLE 2 
     Treatment of a uraniferous liquor 
     The treated liquor had the following composition: 
     
         ______________________________________                                    
Na.sub.2 SO.sub.4      40.1 g/l                                           
Na.sub.2 CO.sub.3      4.93 g/l                                           
NaHCO.sub.3            7.47 g/l                                           
Na.sub.4 UO.sub.2 (CO.sub.3).sub.3                                        
                       5.76 g/l, namely                                   
                       U = 2.53 g/l                                       
Mo in the form of molybdates                                              
                       130 ppm                                            
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     The resin used was the same as in the preceding example. It had particle sizes ranging from 297 to 1190 microns. It was used in a volume of 92 cm 3 , in a column whose height was 13 cm and diameter 3 cm. 
     The ion exchange was carried out as in the preceding example and the results are expressed in table III hereafter. 
     The results show that the loading capacity of the resin in sodium ions was not altered by the presence of uranium. The residual uranium content of the resin was 11 mg per liter, which represents 0.06% of the amount of uranium which has circulated. The molybdenum was not fixed. 
     The uranium contained in the eluted solution was then separated by heating and refluxing the solution for from 1 to 4 hours, whereby most of the uranium was precipitated from the liquor in the form of ammonium diuranate. 
     The residual liquor was then treated according to the procedure of the preceding example, to effect the precipitation of the residual sulfates and carbonates in the state of calcium sulfates and carbonates. 
     As in the preceding case, the resin was regenerated by eluting it with a solution containing about 100 g per liter of ammonium carbonate. A solution of sodium carbonate was obtained which, after evaporation up to a concentration from 50 to 100 grams of sodium carbonate per liter, provided part of the leaching solution for fresh charges of uraniferous ore. 
     
                       TABLE III                                                   
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           Contents (mg per 1.)                                           
                 Na in the  U on the                                      
                                    Mo on the                             
Nr.   Flow rate  liquor     resin   resin                                 
______________________________________                                    
1     0.65       12.800     7       traces                                
2     1.25       21.900     5       &#34;                                     
3     1.81       28.00      4       &#34;                                     
4     2.35       16.400     4       &#34;                                     
5     2.89       8.550      1       &#34;                                     
6     3.41       3.730      1       &#34;                                     
7     3.91       1.560      &lt;1      &#34;                                     
8     4.40       780        &lt;1      &#34;                                     
9     4.89       330        &lt;1      &#34;                                     
10    5.39       140        &lt;1      &#34;                                     
11    5.89       90         &lt;1      &#34;                                     
12    6.32       80         &lt;1      &#34;                                     
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