Electrochemical process for the recovery of metallic iron and sulfuric acid values from iron-rich sulfate wastes, mining residues and pickling liquors

Abstract

An electrochemical process for the recovery of metallic iron or an iron-rich alloy, oxygen and sulfuric acid from iron-rich metal sulfate wastes is described. Broadly, the electrochemical process comprises providing an iron-rich metal sulfate solution; electrolyzing the iron-rich metal sulfate solution in an electrolyzer comprising a cathodic compartment equipped with a cathode having a hydrogen over-potential equal or higher than that of iron and containing a catholyte having a pH below about 6.0; an anodic compartment equipped with an anode and containing an anolyte; and a separator allowing for anion passage; and recovering electrodeposited iron or iron-rich alloy, sulfuric acid and oxygen gas. Electrolyzing the iron-rich metal sulfate solution causes iron or an iron-rich alloy to be electrodeposited at the cathode, nascent oxygen gas to evolve at the anode, sulfuric acid to accumulate in the anodic compartment and an iron depleted solution to be produced.

1 . An electrochemical process for the recovery of metallic iron or an iron-rich alloy, oxygen and sulfuric acid from an iron-rich metal sulfate solution, said process comprises:
 a) providing an iron-rich metal sulfate solution;   b) electrolyzing said iron-rich metal sulfate solution in an electrolyzer comprising a cathodic compartment equipped with a cathode having a hydrogen over-potential equal or higher than that of iron and containing a catholyte having a pH below about 6.0; an anodic compartment equipped with an anode and containing an anolyte; and a separator allowing for anion passage; and   c) recovering electrodeposited iron or iron-rich alloy, sulfuric acid and oxygen gas;   
       wherein: 
       electrolyzing said iron-rich metal sulfate solution causes iron or an iron-rich alloy to be electrodeposited at the cathode, nascent oxygen gas to evolve at the anode, sulfuric acid to accumulate in said anodic compartment and an iron depleted solution to be produced. 
     
     
         2 . The electrochemical process of  claim 1 , wherein step a) includes leaching an iron rich feedstock to produce a slurry; and subjecting said slurry to a separation step to provide the iron-rich metal sulfate solution. 
     
     
         3 . The electrochemical process of  claim 1 , wherein the iron-depleted solution is replenished with fresh iron-rich metal sulfate solution and recirculated. 
     
     
         4 . The electrochemical process of  claim 1 , further comprising the step of etching the cathode prior to the electrolyzing step. 
     
     
         5 . The electrochemical process of  claim 1 , wherein the pH of the catholyte is adjusted to a pH ranging from about 0.5 to about 6. 
     
     
         6 . The electrochemical process of  claim 1 , wherein the pH of the catholyte is adjusted to a pH ranging from about 1 to about 5. 
     
     
         7 . The electrochemical process of  claim 1 , wherein the pH of the catholyte is adjusted to a pH ranging from about 1.5 to about 4.5. 
     
     
         8 . The electrochemical process of  claim 1 , wherein the pH of the catholyte is adjusted to a pH ranging from about 2 to about 4. 
     
     
         9 . The electrochemical process of  claim 1 , wherein the cathode has an over-voltage at 200 A.m −2  of greater than about 466 mV in 0.5 mol.dm −3  H 2 SO 4  solution at 25° C. and an over-voltage at 1000 A.m −2  of greater than about 800 mV in 1.0 mol.dm −3  H 2 SO 4  solution at 25° C. 
     
     
         10 . The electrochemical process of  claim 1 , wherein the cathode comprises a material or is coated with a material selected from the group consisting of nickel, nickel alloy, iron, iron alloy, titanium, titanium alloy, zirconium, zirconium alloy, zinc, zinc alloy, cadmium, cadmium alloy, tin, tin alloy, copper, copper alloy, lead, lead alloy, niobium, niobium alloy, gold, gold alloy, mercury and a metallic amalgam including mercury. 
     
     
         11 . The electrochemical process of  claim 10 , wherein the cathode comprises or is coated with titanium or titanium alloy. 
     
     
         12 . The electrochemical process of  claim 11 , wherein the titanium alloy is a titanium palladium alloy. 
     
     
         13 . The electrochemical process of  claim 4 , wherein the etching step comprises treating the cathode with an acid. 
     
     
         14 . The electrochemical process of  claim 13 , wherein the acid is selected from the group consisting of oxalic acid and a mixture of fluoro and nitric acid. 
     
     
         15 . The electrochemical process of  claim 1 , wherein the anolyte comprises a sulfuric acid solution. 
     
     
         16 . The electrochemical process of  claim 15 , wherein the sulfuric acid solution comprises a concentration ranging from about 5 to about 60 wt. %. 
     
     
         17 . The electrochemical process of  claim 16 , wherein the sulfuric acid solution comprises a concentration of about 30 wt. %. 
     
     
         18 . The electrochemical process of  claim 15 , wherein the anolyte circulates in a loop within the anodic compartment. 
     
     
         19 . The electrochemical process of  claim 1 , wherein the iron-rich metal sulfate solution acting as the catholyte circulates in a loop within the cathodic compartment. 
     
     
         20 . The electrochemical process of  claim 1 , wherein the anode is a dimensionally stable anode either comprising
 i) a material of the formula M/M x O y -A z O t , wherein:
 a) M is a refractory metal or an alloy with a valve action property; 
 b) M x O y  is a metallic oxide of a valve metal; and 
 c) A z O t  is an electrocatalytic metal oxide of a noble metal, an oxide of the platinum group metals or a metallic oxide; 
   ii) an electronically conductive ceramic material;   iii) a conductive oxide having a spinel structure AB 2 O 4 , wherein
 a) A is selected from the group consisting of Fe(II), Mn(II) and Ni(II); and 
 b) B is selected from the group consisting of Al, Fe(III), Cr(III) and Co(III); 
   iv) a conductive oxide having a perovskite structure ABO 3 , wherein
 a) A is selected from the group consisting of Fe(II), Mn(II), Co(II) and Ni(II); and 
 b) B is Ti(IV); 
   v) a conductive oxide having a pyrochlore structure AB 2 O 7 , wherein
 a) A is selected from the group consisting of Fe(II), Mn(II), Co(II) and Ni(II); and 
 b) B is Ti(IV); 
   vi) a carbon based material; or   vii) a lead or lead alloy.   
     
     
         21 . The electrochemical process of  claim 20 , wherein the refractory metal or the alloy of the refractory metal is selected from the group consisting of titanium, titanium alloy, zirconium, zirconium alloy, hafnium, hafnium alloy, vanadium, vanadium alloy, niobium, niobium alloy, tantalum and tantalum alloy. 
     
     
         22 . The electrochemical process of  claim 20 , wherein the metallic oxide of a valve metal is selected from the group consisting of TiO 2 , ZrO 2 , HfO 2 , NbO 2 , Nb 2 O 5 , TaO 2 , and Ta 2 O 5 . 
     
     
         23 . The electrochemical process of  claim 20 , wherein the oxide of the platinum group metals is selected from the group consisting of RuO 2 , IrO 2  and P t O x . 
     
     
         24 . The electrochemical process of  claim 20 , wherein the metallic oxide is selected from the group consisting of SnO 2 , Sb 2 O 5  or Bi 2 O 3 . 
     
     
         25 . The electrochemical process of  claim 22 , wherein the metallic oxide of a valve metal forms a thin impervious protective layer over the refractory metal or the alloy of the refractory metal. 
     
     
         26 . The electrochemical process of  claim 20 , wherein the ceramic material comprises titanium oxides having the general formula Ti n O 2n−1 , wherein n is an integer equal to or higher than 3. 
     
     
         27 . The electrochemical process of  claim 20 , wherein the carbon based material is selected from the group consisting of graphite, impervious graphite and vitreous carbon. 
     
     
         28 . The electrochemical process of  claim 20 , wherein the lead alloy is selected from the group consisting of lead-silver alloys, lead-tin alloys, lead-antimony alloys, and lead-tin antimony alloys. 
     
     
         29 . The electrochemical process of  claim 1 , wherein step b) is performed in a two-compartment electrolyser comprising an ion exchange membrane separating the anodic compartment from the cathodic compartment. 
     
     
         30 . The electrochemical process of  claim 29 , wherein the ion exchange membrane is an anion-exchange membrane. 
     
     
         31 . The electrochemical process of  claim 1 , wherein step b) is performed in a three-compartment electrolyser comprising a central compartment disposed between the anodic compartment and the cathodic compartment and wherein an ion exchange membrane separates the anodic and cathodic compartments from the central compartment. 
     
     
         32 . The electrochemical process of  claim 31 , wherein an anion exchange membrane separates the anodic compartment from the central compartment and wherein a cation exchange membrane separates the cathodic compartment from the central compartment. 
     
     
         33 . The electrochemical process of  claim 32 , wherein the iron-rich metal sulfate solution is circulated within the central compartment. 
     
     
         34 . The electrochemical process of  claim 31 , wherein the catholyte circulates in a loop within the cathodic compartment. 
     
     
         35 . The electrochemical process of  claim 34 , wherein the catholyte comprises an iron (II) sulfate heptahydrate solution. 
     
     
         36 . The electrochemical process of  claim 35 , wherein the catholyte comprises an iron (II) sulfate heptahydrate solution having a concentration ranging from about 1 to about 800 g/L. 
     
     
         37 . The electrochemical process of  claim 36 , wherein the catholyte comprises an iron (II) sulfate heptahydrate solution having a concentration of about 600 g/L. 
     
     
         38 . The electrochemical process of any one of  claim 18 ,  19 , or  34 , wherein the anolyte or catholyte comprises a flow rate ranging from about 0.1 L/min to about 100 L/min. 
     
     
         39 . The electrochemical process of  claim 38 , wherein the flow rate ranges from about 0.1 L/min to about 30 L/min. 
     
     
         40 . The electrochemical process of  claim 39 , wherein the flow rate is about 2 L/min. 
     
     
         41 . The electrochemical process of  claim 1 , wherein step b) is performed under constant current and at a current density ranging from about 50 to about 10000 A/m 2 . 
     
     
         42 . The electrochemical process of  claim 41 , wherein the current density ranges from about 50 to about 5000 A/m 2 . 
     
     
         43 . The electrochemical process of  claim 42 , wherein the current density is about 2500 A/m 2 . 
     
     
         44 . The electrochemical process of  claim 41 , wherein the current density ranges from about 3000 to about 5000 A/m 2 . 
     
     
         45 . The electrochemical process of  claim 44 , wherein the current density is about 4000 A/m 2 . 
     
     
         46 . The electrochemical process of  claim 41 , wherein the current density ranges from about 5000 to about 10000 A/m 2 . 
     
     
         47 . The electrochemical process of  claim 46 , wherein the current density is about 7000 A/m 2 . 
     
     
         48 . The electrochemical process of  claim 1 , wherein step b) is performed at a temperature ranging from about 20 to about 100° C. 
     
     
         49 . The electrochemical process of  claim 48 , wherein the temperature ranges from about 30° C. to about 70° C. 
     
     
         50 . The electrochemical process of  claim 49 , wherein the temperature is about 50° C. 
     
     
         51 . The electrochemical process of  claim 1 , wherein the recovered nascent oxygen gas is further dried and liquefied. 
     
     
         52 . The electrochemical process of  claim 1 , wherein the recovered sulfuric acid is concentrated and/or recirculated.