Electrochemical process for the recovery of metallic iron and chlorine values from iron-rich metal chloride wastes

Abstract

An electrochemical process for the concurrent recovery of iron metal and chlorine gas from an iron-rich metal chloride solution, comprising electrolysing the iron-rich metal chloride solution in an electrolyser comprising a cathodic compartment equipped with a cathode having a hydrogen overpotential higher than that of iron and containing a catholyte having a pH below about 2, an anodic compartment equipped with an anode and containing an anolyte, and a separator allowing for anion passage, the electrolysing step comprising circulating the iron-rich metal chloride solution in a non-anodic compartment of the electrolyser, thereby causing iron to be electrodeposited at the cathode and chlorine gas to evolve at the anode, and leaving an iron-depleted solution. The iron-rich metal chloride solution may originate from carbo-chlorination wastes, spent acid leaching liquors or pickling liquors.

1 . An electrochemical process for the recovery of metallic iron and chlorine gas from an iron-rich metal chloride solution, which process comprises:
 a) providing an iron-rich metal chloride solution;   b) electrolysing said iron-rich metal chloride solution in an electrolyser comprising a cathodic compartment equipped with a cathode having a hydrogen overpotential higher than that of iron and containing a catholyte having a pH below about 2, an anodic compartment equipped with an anode and containing an anolyte, and a separator allowing for anion passage, said electrolysing step comprising circulating said iron-rich metal chloride solution in a non-anodic compartment of said electrolyser, thereby causing iron to be electrodeposited at the cathode and chlorine gas to evolve at the anode, and leaving an iron-depleted solution; and   c) separately recovering said electrodeposited iron and said chlorine gas.   
   
   
       2 . The electrochemical process of  claim 1 , wherein step a) of providing an iron-rich metal chloride solution includes the following steps:
 a1) leaching a solid carbo-chlorination waste with a hot aqueous solution, thereby forming an aqueous slurry; and   a2) subjecting said aqueous slurry to a separation of solids, thereby forming an insoluble cake and isolating an iron-rich metal chloride solution.   
   
   
       3 . The electrochemical process according to  claim 1 , wherein the pH of the catholyte is adjusted to range between about 0.3 and about 1.8, preferably between about 0.6 and about 1.5, more preferably between about 0.6 and about 1.1, most preferably between about 0.9 and about 1.1. 
   
   
       4 . The electrochemical process according to  claim 1 , wherein the cathode has an overvoltage, at 200 A.m −2 , greater than about 425 mV in 0.5 mol.dm −3  HCl at 25° C. 
   
   
       5 . The electrochemical process according to  claim 4 , wherein the cathode is constructed from or coated with a material selected from the group consisting of 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 metallic amalgam with mercury. 
   
   
       6 . The electrochemical process according to  claim 5 , wherein the material consists of titanium or titanium alloy, preferably titanium palladium ASTM grade 7. 
   
   
       7 . The electrochemical process according to  claim 1 , wherein the cathode is pretreated before the electrolysing step by acid etching and thorough rinsing with deionised water to eliminate traces of acid. 
   
   
       8 . The electrochemical process according to  claim 1 , wherein said anolyte is circulated in loop within the anodic compartment of the electrolyser. 
   
   
       9 . The electrochemical process according to  claim 1 , wherein said anolyte comprises HCl, preferably about 10 to about 37 wt. %, more preferably about 20%, a salt selected from the group consisting of MgCl 2 , NaCl, LiCl, KCl, CaCl 2  and mixtures thereof, preferably 1 to about 20 wt. %, more preferably about 16 wt. %, and Fe(III) as a corrosion inhibitor, preferably 10 to about 12,000 ppm wt, more preferably about 8000 to about 10000 ppm wt. 
   
   
       10 . The electrochemical process according to  claim 1 , wherein the anode is a dimensionally stable anode of the type [M/M x O y -A z O t ], wherein M is a refractory metal or an alloy with a valve action property, including titanium, titanium alloy, zirconium, zirconium alloy, hafnium, hafnium alloy, vanadium, vanadium alloy, niobium, niobium alloy, tantalum or tantalum alloy, wherein M x O y  is a metallic oxide of a valve metal forming a thin and impervious layer protecting the base metal, including TiO 2 , ZrO 2 , HfO 2 , NbO 2 , Nb 2 O 5 , TaO 2 , or Ta 2 O 5 , and wherein A z O t  is an electrocatalytic metal oxide of a noble metal, an oxide of the platinum group metals including RuO 2 , IrO 2  or P t O x , or a metallic oxide, including SnO 2 , Sb 2 O 5  or Bi 2 O 3 . 
   
   
       11 . The electrochemical process according to  claim 1 , wherein the anode is constructed from bulk electronically conductive ceramics, including sub-stoichiometric titanium oxides having as a general formula Ti n O 2n-1 , wherein n is an integer equal to or above 3; conductive oxides with a spinel structure AB 2 O 4 , wherein A is Fe(II), Mn(II) or Ni(II), and B is Al, Fe(III), Cr(III) or Co(III); or conductive oxides with a perovskite structure ABO 3 , wherein A is Fe(II), Mn(II), Co(II) or Ni(II), and B is Ti(IV) or with a pyrochlore structure AB 2 O 7 . 
   
   
       12 . The electrochemical process according to  claim 1 , wherein the anode is constructed from carbon-based materials such as graphite, impervious graphite, or vitreous carbon. 
   
   
       13 . The electrochemical process according to  claim 1 , wherein the electrolysing step is performed in a two-compartment electrolyser in which the separator is an ion-exchange membrane, preferably an anion-exchange membrane, and wherein said iron-rich metal chloride solution is circulated in loop within the cathodic compartment of the electrolyser, acting as the catholyte. 
   
   
       14 . The electrochemical process according to  claim 13 , wherein the iron-rich metal chloride solution is adjusted to a pH below 2. 
   
   
       15 . The electrochemical process according to  claim 1 , wherein the electrolysing step is performed in a three-compartment electrolyser in which the anodic and cathodic compartments are separated from a central compartment by an anion and a cation exchange membranes, respectively, and wherein the iron-rich metal chloride solution is circulated within the central compartment of the electrolyser. 
   
   
       16 . The electrochemical process according to  claim 15 , wherein said catholyte is circulated in loop within the cathodic compartment. 
   
   
       17 . The electrochemical process according to  claim 16 , wherein the catholyte comprises about 1 to about 450 g/L of iron (II) chloride, preferably about 335 g/L, about 1 to about 350 g/L MgCl 2  or CaCl 2  or a mixture thereof, preferably about 250 g/L, preferably MgCl 2 , and 0 to about 10 g/L of free HCl. 
   
   
       18 . The electrochemical process according to  claim 1 , wherein a volume flow rate of both anolyte and catholyte ranges between about 0.1 L/min and about 100 L/min. 
   
   
       19 . The electrochemical process according to  claim 1 , wherein the electrolysing step is performed under constant current at a current density ranging from about 50 to about 5000 A/m 2 . 
   
   
       20 . The electrochemical process according to  claim 19 , wherein the electrolysing step is performed under constant current at a current density ranging from about 50 to about 1000 A/m 2 , preferably about 500 A/m 2 , thereby obtaining an essentially dendrite-free smooth deposit of iron. 
   
   
       21 . The electrochemical process according to  claim 19 , wherein the electrolysing step is performed under constant current at a current density ranging from about 3000 to about 5000 A/m 2 , thereby obtaining an essentially powdered iron. 
   
   
       22 . The electrochemical process according to  claim 1 , wherein the electrolysing step is performed at an operating temperature ranging from about 40 to about 110° C. 
   
   
       23 . The electrochemical process according to  claim 1 , wherein the iron-rich metal chloride solution originates from carbo-chlorination wastes, spent acid leaching liquors or pickling liquors. 
   
   
       24 . The electrochemical process according to  claim 1 , wherein the iron-rich metal chloride solution comprises vanadium, said process further comprising a vanadium separation step before, during or after the electrolysing step. 
   
   
       25 . The electrochemical process according to  claim 24 , wherein said vanadium separation step occurs before the electrolysing step. 
   
   
       26 . The electrochemical process according to  claim 25 , wherein said vanadium separation step consists in removing vanadium from the iron-rich metal chloride solution concurrently with chromium by co-precipitation at a pH ranging from about 0.5 to about 3.0. 
   
   
       27 . The electrochemical process according to  claim 24 , wherein the pH of the catholyte ranges between about 0.3 and about 0.5, causing vanadium to precipitate at the cathode along with iron electrodeposition, and wherein the vanadium-separation step occurs after the electrolysing step. 
   
   
       28 . The electrochemical process according to  claim 24 , wherein the pH of the catholyte ranges between about 0.6 and about 1.8, causing vanadium to essentially remain within the circulating iron-rich metal chloride solution while iron is electrodeposited at the cathode, and wherein vanadium is thereafter recovered from the iron-depleted solution exiting the electrolyser, whereby the vanadium separation step occurs during the electrolysing step. 
   
   
       29 . The electrochemical process according to  claim 1 , wherein chlorine gas recovered from the anode is further dried and liquefied. 
   
   
       30 . The electrochemical process according to  claim 1 , wherein the iron-depleted solution exiting the electrolyser is recovered and further treated in order to remove calcium and radioactivity by addition of sulphuric acid, thereby producing a magnesium- and aluminum-rich brine. 
   
   
       31 . The process according to  claim 30 , further comprising a step of pyrohydrolysis of said magnesium- and aluminum-rich brine in a fluid-bed pyrohydrolyser, thereby producing azeotropic hydrochloric acid and spinel beads. 
   
   
       32 . The process according to  claim 31 , further comprising recovery of said azeotropic hydrochloric acid for export. 
   
   
       33 . The process according to  claim 2 , wherein leaching is performed with hot process water, hot diluted hydrochloric acid, hot spent leaching acid or spent pickling liquors. 
   
   
       34 . The process according to  claim 2 , wherein the solid separation step is performed by physical separation method, preferably by decantation, filtration or centrifugation. 
   
   
       35 . An electrochemical process for the recovery of metallic iron and chlorine gas from an iron-rich metal chloride solution, which process comprises:
 a) providing an iron-rich metal chloride solution;   b) electrolysing said iron-rich metal chloride solution in a two-compartment electrolyser comprising a cathodic compartment equipped with a cathode having a hydrogen overpotential higher than that of iron, and an anodic compartment equipped with an anode and containing an anolyte, said cathodic and anodic compartments being separated by an anion-exchange membrane, said electrolysing step comprising circulating said iron-rich metal chloride solution, adjusted to a pH below 2, as a catholyte in said cathodic compartment of said electrolyser, thereby causing iron to be electrodeposited at the cathode and chlorine gas to evolve at the anode, and leaving an iron-depleted solution; and   c) separately recovering said electrodeposited iron and said chlorine gas.   
   
   
       36 . An electrochemical process according to  claim 35  wherein in step c) recovering iron is conducted by physically stripping said iron electrodeposited at the cathode and recovering chlorine is conducted by suctioning of chlorine gas above the anodic compartment. 
   
   
       37 . The electrochemical process according to  claim 2 , wherein the pH of the catholyte is adjusted to range between about 0.3 and about 1.8, preferably between about 0.6 and about 1.5, more preferably between about 0.6 and about 1.1, most preferably between about 0.9 and about 1.1. 
   
   
       38 . The electrochemical process according to  claim 8  wherein the acid etching is done by immersion into fluoro-nitric acid mixture having the following composition: about 70 vol % conc. HNO 3 , about 20 vol. % conc. HF and about 10 vol. % H 2 O. 
   
   
       39 . The electrochemical process according to  claim 16  wherein the pH ranges from about 0.3 to about 1.8. 
   
   
       40 . The electrochemical process according to  claim 16  wherein the pH ranges from about 0.6 to about 1.1.