Abstract:
Metal hydroxides are produced in an easy-to-separate powder form from metal in solution, by passing an electric current through the solution to produce the formation of a preceipitated hydroxide against a solid ion-exchange membrane, which membrane separates the anode compartment from the cathode compartment. When the solution is acidic, the membrane is an anion exchange membrane. When the solution is basic, the membrane is a cation exchange membrane.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of producing metal hydroxides in an easy-to-separate powder form, as well as to various applications of this method. 
     2. Discussion of Background Information 
     Metal hydroxides of the general formula Me (OH) n  are usually prepared by the action of an alkaline solution on soluble metal salts, to precipitate hydroxides which have a finely divided and often a gelatinous appearence. 
     The gelatinous appearance obtained does not facilitate washing or the hydroxide and separating it by filtration from the starting solution and from the washing waters. It is therefore difficult to obtain the hydroxide in a powder form with an acceptable purity. 
     In some other cases, in particular when the hydroxides are soluble in an alkali medium, for example the hydroxides of amphoteric metals, they may in theory be obtained as precipitates by neutralizing a strong base with an acid. In many practical applications, the metals which are dissolved in strong base media are not recovered, but the solutions containing them are purely and simply discharged, with the consequent risks of pollution and toxicity. 
     SUMMARY OF THE INVENTION 
     The present invention aims to remedy these drawbacks and to produce metal hydroxides both from acid solutions and from alkaline solutions, in a finely divided and easy-to-wash form and, consequently, with a high degree of purity. 
     This method is characterized in that an electric current is passed through the solution in which the metal is dissolved to produce the formation of a precipitated hydroxide against a solid ion-exchange membrane, which membrane separates the anode compartment from the cathode compartment. 
     The expression &#34;against a solid ion-exchange membrane&#34; as used above means that the precipitation takes place on the membrane, or in the immediate proximity thereof, in a zone of the order of 1 mm thick from the membrane. 
     According to a first embodiment, the solution is an acid solution and the membrane is an anionic membrane, for example a membrane comprising quaternary ammonium groups. This embodiment enables the precipitation and the separation of the metal hydroxide from an acid solution. 
     According to a second embodiment, the solution is a basic solution and the membrane is a cationic membrane, for example a membrane comprising SO 3  H- groups. 
     In either case, the membrane is for example an insoluble polymer incorporating an ion-exchange resin, or an insoluble polymer, for example polytetrafluoroethylene, which has been irradiated in a manner to graft polystyrene carriers of charged groups, as those indicated above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood with reference to the accompanying drawings, given by way of non limiting example. In the drawings: 
     FIG. 1 corresponds to the embodiment where the metal hydroxide is precipitated from an acid solution, and 
     FIG. 2 corresponds to the embodiment wherein the metal hydroxide is precipitated from an alkali solution. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a vat 1 in which an electric current is to be passed, and which is divided into two parts, a cathode compartment 2 and an anode compartment 3, by an anionic membrane 4. In this vat, a cathode 5 is placed in the cathode compartment 2 and an anode 6 is placed in the anode compartment 3. 
     The cathode compartment 2 is filled with a catholyte, for example a basic solution of caustic soda or caustic potash, and it is fitted with a cathode 5 made of a metal stable in alkali media, for example nickel. 
     The anode compartment 3 is filled with anolyte, namely a solution of the metal Me whose hydroxide is to be precipitated. The anode is for example made of the same metal, employing the soluble anode technique. 
     An electric current is caused to flow, advantageously at a potential difference comprised between 5 and 20 V, at a current density preferably comprised between 5 and 20 A/dm 2 , which produces the following movements. 
     The metal ions Me n+  in the anolyte move towards the cathode, but they are prevented from passing into the catholyte by the anion-exchange membrane; the OH -  ions of the catholyte move towards the anode, pass through the anion-exchange membrane and come into contact with the ions Me n+ . 
     The formation of the hydroxide Me(OH) n  thus takes place on the anionic face of the membrane, because the anionic face is highly basic, which is permanently maintained due to the constant regeneration of the membrane by the continuous supply of OH -  ions. 
     The hydroxide formed is detached from the membrane and sinks in the anolyte. It has a powdered and dry appearance enabling it to be filtered and washed without any difficulty. 
     For the formation of the hydroxide, it is important that the anolyte be maintained at a pH comprised between 0.5 and a pH less than that which would cause hydrolysis of the anolyte solution, which would lead to an unwanted precipitation of the hydroxide in a gelatinous form. This pH is for example of the order of 4.5 when the anolyte is a ZnSO 4  solution, in which case the precipitate obtained is obviously Zn(OH) 2 . The concentration of metal Me in the anolyte is maintained constant by dissolution of the anode. 
     In a variation, instead of operating in the soluble anode mode, the concentration of the treated solution is reconstituted by the addition and dissolution of a salt of the metal whose hydroxide is to be produced, for example a carbonate. A non-soluble anode, for example of lead or ruthenised titanium is used. 
     It is also possible to produce hydroxides of metals such as chromium, nickel, cadmium, cobalt, zinc or uranium, or double or triple hydroxides such as nickel-cadmium hydroxide, or nickel-cadmium-cobalt hydroxide. 
     The applications of this method concern notably the treatment of uranium ore to recover the metal through its hydroxide, the starting acid solution containing the uranium salt being an acid feed solution of an uranium ore. 
     FIG. 2 shows a similar installation, comprising a vat 1, a cathode compartment 2 with a cathode 5 and an anode compartment 3 with an anode 6. In this instance, the cathode compartment 2 and the anode compartment 3 are separated by a cation-exchange membrane 7. 
     The anode compartment is filled with the solution to be treated, or anolyte, from which the hydroxide is to be precipitated and which, it should be recalled, is a solution of a metal in an alkaline medium. Advantageously a highly concentrated solution of caustic soda or caustic potash is used, for example 8N caustic soda or caustic potash. The cathode compartment is filled with the catholyte, for example a 0.5N solution of caustic potash and the anode and cathode compartments are fitted with respective electrodes of dimensionally-stable, insoluble metal, for instance as mentioned above. 
     The flow of electric current induces the following movements. 
     The alkaline cations, for example Na +  or K + , move from the anode compartment 3 towards the cathode compartment 2 through the cation-exchange membrane 7. The catholyte becomes enriched with alkali as the flow of current continues, and, consequently, the pH of the anolyte is reduced. When the pH has dropped to a sufficiently low value, the metal hydroxide Me(OH) n  precipitates in an easy-to-filter form. We may thus refer to this as an electro-dialysis phenomenom. 
     An advantageous application of this embodiment is the regeneration of strong basic solutions obtained during electrochemical forming of metals, for example, of aluminium. In this case, the starting solution containing aluminium in the form of AlO 2   -  ions has a concentration of 8N, and its properties deteriorate when the concentration drops to a value of about 2N. At this point, an 8N alkaline solution has to be reformulated in the cathode compartment, and aluminium recovered in the form of hydroxide precipitated in the anode compartment. 
     Another application is the regeneration, by the recovery of metals dissolved therein, of basic solutions, namely basic solutions of batteries and accumulators, for example aluminium/air batteries. 
     The invention will now be illustrated by several non-limiting examples. 
     EXAMPLES 1 to 5 
     Using the conditions summarized on the following table where percentages (%) are indicated by weight, hydroxides are obtained in a easy-to-filter and purified form, it being understood that these hydroxides due to their powder form may easily undergo later purification cycles. 
     
         __________________________________________________________________________PreparedExampleHydroxide  Anolyte Catholyte                           Current                                  Product                                         Efficiency__________________________________________________________________________1    Nickel Hydroxide           22 N nickel                   N caustic                           10 A/dm2                                  Nickel 65 to 85%           sulfate soda    V 12 to 18 V                                  hydroxide           solution                   solution       with 70%           Nickel anode                   Nickel         nickel                   cathode2    Cadmium Hydroxide           12 N cadmium                   N caustic                           10 A/dm2                                  Cadmium                                         70 to 80%           sulfate soda    V 15 to 19 V                                  hydroxide           Solution                   solution       with 81.5%           Cadmium Nickel         cadmium           anode   cathode3    Nickel and 12 N nickel                   N caustic                           10 A/dm2                                  NickelCadmium hydroxide           sulfate soda    V 12 to 18 V                                  cadmium           solution                   solution       hydroxide           0.3 N cadmium                   Lead cathode   comprising           sulfate                67.6% Ni,           solution               3.3% Cd           Cd/Ni 5%               Cd/Ni 5%           Nickel anode4    Triple Hydroxide           12 N nickel                   N caustic                           10 A/dm2                                  Triple 65 to 85%of nickel, sulfate soda    V 12 to 18 V                                  hydroxide ofcadmium and           solution                   solution       nickel,cobalt     0.3 N cadmium                   Nickel         cadmium and           sulfate cathode        cobalt           solution               comprising           Cd/Ni 5%               62.6% Ni           1.2 N cobalt           3% Cd           sulfate                6.2% Co           Co/Ni 10%              Cd/Ni 5%           Nickel anode    Co/Ni 10%5    Aluminium  2 N caustic                   0.5 N caustic                           15 A/dm2                                  AluminiumHydroxide  potash  potash  V 6 to 15 V                                  hydroxide           Potassium                   Lead cathode   comprising           aluminate              39%           495 g/l                aluminium in           Lead anode             the dry                                  product of                                  the anolyte                                  7 N caustic                                  potash in                                  the catholyte__________________________________________________________________________