MEMBRANE ELECTROLYSIS APPARATUS AND METHOD OF REMOVING METAL IONS USING SUCH APPARATUS

Abstract: A membrane electrolysis cell for the removal of metal ions from an aqueous solution has an anode compartment (1), and a cathode compartment (2) containing the aqueous solution, a semi-permeable membrane (6) arranged as a wall separating the anode and cathode compartments. To avoid the need to replenish the anode solution, the anode (4) is consumable, e.g. is of iron, and the membrane (6) is anion-exchanging, to permit anions to pass from the cathode compartment to the anode compartment.

Description: The invention relates to membrane electrolysis apparatus and to use of such apparatus for the removal of metal ions from an aqueous solution, especially the removal of unwanted metal ions from aqueous waste solutions. A known membrane electrolysis cell for the removal of a metalliferous contamination from an aqueous solution has at least two compartments, an anode, a cathode and a semipermeable membrane located between the two compartments, wherein a first of the two compartments is provided with the anode and the second compartment is provided with the cathode and wherein the second compartment is suitable for holding an aqueous solution. Such a membrane electrolysis apparatus is described for example in PT-Procestechniek 42 (1987), no. 12, page 123. In the known apparatus the aqueous solution is fed for example to the second compartment in a continuous process in the form of a waste flow. In the waste flow salts of for example Zn, Pb, Ni or Cu are dissolved, and it is desired to remove the cations before disposing of the waste solution further. In the first compartment there is the anode, often of stainless steel, and for example a 4% solution of NaOH. Between the first and second compartments there is a cation exchanging membrane, for example of Nafion (trade mark of Du Pont de Nemours). The following problems arise with this known apparatus. During operation the sodium ions from the first compartment serve to maintain the electric current between anode and cathode. These sodium ions travel through the membrane from the first to the second compartment. The hydroxide ions remaining in the first compartment dissociate under the effect of the electric current and so oxygen is evolved. Consequently the sodium hydroxide concentration needs to be replenished constantly in order that the electric current may be maintained. The transported sodium ions cause an increased concentration in the second compartment. In order to prevent this leading to deposition of sodium salts, the aqueous solution in the second compartment needs to be continually changed. However, discharging the waste flow from the second compartment is problematical on account of the environmental effects associated with it. The membrane electrolysis apparatus and the method in accordance with the invention are characterised in that the anode is a consumable metal anode and the semi-permeable membrane is an anion-exchanging membrane. In the electrolysis, the consumable anode is dissolved and metal ions in the solution contacting the cathode are deposited at the cathode. The metal of the anode may be the same as or different from the metal of the ions deposited. Suitable anodes are of Fe, Cu and Ni, and are preferably highly pure. It is preferable that when the anode is of iron a solution of FeCl₂ is introduced into the first compartment. Iron is especially preferred as the anode. With the invention maintenance of the electric current is by means of the negative ions of the salts whose metal is to be removed from the aqueous solution, for example chloride ions. To this end the membrane needs to be an anion-exchanging membrane, for example a Neosepta membrane (Neosepta is a trade mark of Tokuyama Soda Company Ltd., Japan). The chloride ions thus meet the metal ions in the anode compartment, which originate from the anode positioned in it, to form a salt. This salt may be recoverable as a useful product, e.g. if it is FeCl₂ which is a product usable as coagulant. Consequently the invention can have an environmental advantage over the known apparatus, meaning that whereas the known apparatus produces environmentally harmful residual salts, use of the apparatus in accordance with the invention can produce an environmentally friendly, reusable residual product. There is also an advantage in operating economy; for maintenance of the electric current there is no need for continuous replenishment of the solution in the anode compartment as is the case with the known apparatus. Indeed with the invention the ion supplying the current is obtained from the solution from which metal ions are to be removed. The invention will now be illustrated by reference to the accompanying drawing, in which: Fig. 1 shows diagrammatically a membrane electrolysis apparatus in accordance with the prior art. Fig. 2 shows diagrammatically a membrane electrolysis apparatus in accordance with the present invention. In the two figures, the same reference numerals indicate corresponding parts. Fig. 1 and 2 show the prior art apparatus and an apparatus of the invention respectively, each comprising an electrolysis cell 3 which has a first (anode) compartment 1 and a second (cathode) compartment 2 in which respectively an anode 4 and a cathode 5 are located. A semipermeable membrane 6 separates the first compartment 1 from the second compartment 2. The aqueous solution to be cleaned is in the second compartment 2 and is typically a waste solution containing a contaminant in the form of a metal salt. With the known apparatus of Fig. 1, the anode 4 is of stainless steel and the membrane 6 is a cation exchanging membrane of for example Nafion. A 4% sodium hydroxide solution is introduced into the anode compartment 1 and the electric current flow is supplied by transport of sodium ions from the solution in compartment 1, through the membrane 6 to compartment 2. In both the apparatus in Fig. 1 and that in Fig. 2, the cathode 5 is typically of zinc, but other suitable materials may also be used. The inconvenience with the known apparatus of Fig. 1 is that the sodium hydroxide solution needs to be constantly replenished and that the sodium ions accumulate in the cathode compartment 2 in salt form. These salts have to be removed from that compartment, but discharging them is environmentally harmful. With the apparatus of Fig. 2 embodying the invention the membrane 6 is an anion-exchanging membrane, for example a Neosepta membrane, and the anode 4 is a consumable anode of highly pure iron. Zinc chloride has to be removed from the aqueous solution which can be fed continuously into the cathode compartment 2 in a continuous operation as a waste flow, then the current transport is achieved in the electrolysis cell 3 by the chloride ions. When in the anode compartment 1 these chloride ions recombine with the dissolved iron ions originating from anode 4 into ferrous chloride. This ferrous chloride is, when collected, not harmful to the environment and may later find use as coagulant. Zinc is deposited at the cathode. This process is started by introducing, for example a 4% FeCl₂ solution into the solution in compartment 1. During the further processing the solution does not need to be replenished. The electrolysis is performed by applying a voltage at an appropriate level between the anode and the cathode to cause the following reactions:- At the anode: Fe → Fe⁺⁺ + 2e⁻ At the cathode: Zn⁺⁺ + 2e⁻ → Zn An economic advantage of the invention is apparent from the following example. Example Salts of Ni are to be removed from a waste flow. 737.5 gr Ni can be deposited by 1 kg NaOH in the conventional process using NaOH. Consequently at a NaOH cost price (1989) of Fl 0.80 per kg, recovering 1 kg of Ni costs Fl 1.08 with the conventional method. In accordance with the invention approximately 0.94 kg Fe is needed to recover 1 kg Ni, at a cost price of approximately Fl 1.-/kg (1989). Therefore, recovering 1 kg Ni costs Fl 0.94 with the method in accordance with the invention.

Claims: A membrane electrolysis apparatus for the removal of metal ions from an aqueous solution, comprising an electrolysis cell having an anode compartment (1), a cathode compartment (2), an anode (4) located to contact liquid in said anode compartment, a cathode (5) located to contact said aqueous solution in said cathode department, and a semi-permeable membrane (6) arranged as a wall separating said anode and cathode compartments, characterized in that said anode (4) is a consumable metal anode and said membrane (6) is an anion-exchanging membrane adapted to permit anions to pass from said cathode compartment to said anode compartment. A membrane electrolysis apparatus according to claim 1 wherein said anode is made of Fe, Cu or Ni. A membrane electrolysis apparatus according to claim 1 wherein said anode is made of iron and said anode compartment (1) contains a solution comprising FeCl₂. A method of removing metal ions from a waste aqueous solution comprising subjecting said solution to electrolysis with said solution in contact with a cathode (5) and said solution separated by a semi-permeable membrane (6) from an anode solution in contact with an anode (4) characterized in that said anode (4) is a consumable metal anode and said semi-permeable membrane (6) is anion-exchanging. A method according to claim 4 wherein said anode is made of iron. A method according to claim 4 wherein said anode solution contains FeCl₂. A method according to claim 4 wherein said metal ions are selected from Zn, Pb, Ni and Cu ions.

Inventors: RIJKHOF, EVERT JAN, THOLEN, JOHANNES PETRUS PAULUS, IR. DRS.
Applicants: INDUSTRIAL CONSULTANTS HOOGOVENS B.V.
Application year: 1990
Main classification: C02F1
IPC codes: C02F1, C25C7