Process for Recovery of Waste H.sub.2 SO.sub.4 and HCl

This invention relates to a process for recovery of waste H.sub.2 SO.sub.4 and HCl which are used for pickling metallic materials and articles and contain a large quantities of Fe ions. Furthermore, it offers a process for recovery H.sub.2 SO.sub.4 and HCl from waste H.sub.2 SO.sub.4 which contains a large amounts of Fe ions after removal of Ti ions by a hydrolysis process in the production of TiO.sub.2 and also the recovery of high-purity electrolytic iron or iron hydroxide.

There are two conventional methods of acid recovery. The first method is a crystallization method which crystallizes Fe ions contained as their sulfate or chloride salt by cooling or concentrating waste acids with heat. However, this method has the following disadvantages: (1) large amounts of energy are required for the crystallization; (2) the removal of heavy metallic ions, such as, Fe ions from the waste acids is difficult; (3) an economical method for treating the crystals removed is not available.

The second method is a thermal decomposition method which sprays the waste acids, including Fe ions, into a furnace heated from 600.degree. to 1000.degree. C. The HCl or H.sub.2 SO.sub.4 is recovered by absorbing the gases decomposed into the forms of HCl gas, Cl.sub.2 gas and Fe.sub.2 O.sub.3 or the forms of H.sub.2 SO.sub.4 gas, SO.sub.2 gas, SO.sub.3 gas and Fe.sub.2 O.sub.3, and these are reused for pickling and dissolution of raw materials. The following disadvantages are found in this method. (1) a large energy loss for thermal decomposition in the case of lower Fe ions concentration in the waste acids; (2) an abundant hydrochloric acid loss as HCl fume unless the total Cl.sup.- ions concentration in the pickling process is increased; (3) beating of the pickling solution required to raise the Fe ions concentration in it; (4) difficulty in controlling the Fe concentration in practice, and (5) a low operating rate and high maintenance cost based on severe damage of the apparatus by wet Cl.sub.2 and HCl gases.

Moreover, many methods which recover HCl or H.sub.2 SO.sub.4 by removal of Fe ions in the waste acids as electrolytic iron by electrolysis have been published. These methods introduce the waste HCl or H.sub.2 SO.sub.4 including abundant Fe ions into the cathode compartment in electrolysis process, electrodeposits of Fe metal on the cathode, and at the same time transfer Cl.sup.- or SO.sub.4.sup.2- ions through the anion exchange membrane which divides the cathode and central compartments, and recycle the waste acids which having passed the cathode compartment into the used parts through the central compartment. Abundant free acids in the waste acid in the electrolysis process yield a low Fe electrolysis efficiency, because of the decomposition to H.sup.+ and Cl.sup.- ions or H.sup.+ and SO.sub.4.sup.2- ions (see FIG. 14).

To overcome this disadvantage, the conventional method has proposed to set the diffusion-dialysis membrane prior to the electrolysis process and decrease the amounts of free acid as possible. However, the above method can not improve the relationship of the increasing amounts of free acid attended by electrodeposition of Fe metals due to the direct electrolysis. Furthermore, since the waste acids often contain the heavy metallic ions except Fe ions, only a poor-purity electrolytic iron is obtained in the electrolysis process and consequently high additional valuable materials can not be obtained.

The present invention has overcome the disadvantages of the conventional process described above and relates to a process for treating waste H.sub.2 SO.sub.4 or HCl containing a large amounts of Fe ions and particularly obtaining reusable H.sub.2 SO.sub.4 or HCl and high-purity electrolytic iron or iron hydroxide using both solvent extraction and diaphragm-electrolysis techniques. In general, since most of Fe ions contained in the waste acids are present as Fe.sup.2+ ions, the oxidation method of Fe.sup.2+ ions to Fe.sup.3+ ions with high pressured air or oxygen, or H.sub.2 O is often used.

This invention comprises the following sequential steps:

(1) The first step in which the waste acids containing Fe.sup.2+ ions are introduced into the anode compartment, a solution containing Fe.sup.3+ ions are passed through the cathode compartment divided with diaphragms, and the oxidation-reduction cell is constructed by connecting the electrodes set in both compartments with a wire and Fe.sup.2+ ions in the waste acids are converted to Fe.sup.3+ ions.

(2) The second step in which Fe.sup.3+ ions in the resultant aqueous solution from the first step are extracted with contact of an organic solvent (A) selected from di-alkyl phosphoric acids, for example, D2EHPA (Di-2-ethyl hexyl phosphoric acid) or HDBP (Mono-dodecyl phosphoric acid).

(3) The aqueous solution from the second step in which Fe ions are diminished is recycled to the using parts.

(4) The organic solvent (A) is regenerated by stripping Fe.sup.3+ ions extracted into the organic solvent (A) in the second step by contact with HCl.

(5) The third step in which Fe.sup.3+ ions transferred into HCl strip solution are extracted into the organic phase as ferric chloride complex with contact of an organic solvent (B) containing a phosphoric acid ester or a primary, secondary, tertiary or quarternary amine.

(6) The strip solution in which total Cl.sup.- ions concentration was decreased due to the extraction as ferric chloride complex is introduced into the central compartment of an electro-bath in a fourth step. Then hydrochloric acid is recovered from Cl.sup.- and H.sup.+ ions transferred through the diaphragms and recycled for stripping the organic solvent (A).

(7) The ferric chloride complex extracted into the organic solvent (B) is stripped from the organic solvent (B) with contact of water or one part of catholyte in the electro-bath for Fe recovery in the fourth step. Consequently the organic solvent (B) is regenerated and recycled to extract the ferric chloride complex in the third step.

(8) The solution containing Fe.sup.3+ ions stripped into water or catholyte in the third step is introduced into the cathode compartment in the first step. The oxidation-reduction cell is constructed between the above solution in the cathode compartment and the waste acid in the anode compartment containing Fe.sup.2+ ions by connecting the electrodes set in both compartments with wire and Fe.sup.3+ ions in the strip solution are reduced to Fe.sup.2+ ions. If necessary, an outer cell can be connected in order to conduct completely oxidation-reduction and promote the transfer of free acid produced in the reduction process.

(9) If there are a large quantities of free acid in the aqueous solution reduced to Fe.sup.2+ ions in the first step, the free acid is diminished by diffusion- or electro-dialysis method in the dialysis process and supplied to the cathode compartment of electro-bath for Fe recovery.

(10) CaCl.sub.2 or NH.sub.4 Cl except FeCl.sub.2 is added into the catholyte circulating the cathode compartment in the fourth step owing to the promotion of electrolysis efficiency and the total Cl.sup.- ions concentration in the catholyte is always maintained higher than one in the solution passed through the central compartment.

(11) The anolyte circulating the anode compartment in the fourth step contains 100-400 g/l H.sub.2 SO.sub.4 and feeds H.sup.+ ions to the central compartment.

(12) A conventional porous fine pore membrane, diffusion-dialysis membrane or anion exchange membrane is usable for the diaphragm between cathode and central compartments. While, the diaphragm between anode and central compartments comprises a cation exchange membrane.

(13) To increase total Cl.sup.- ions concentration in the solution passing through the central compartment, more diaphragms of porous fine pore membrane are used to prevent the decrease of selectivity of the cation exchange membrane.

(14) This invention is characterized with the two following recovery methods for H.sub.2 SO.sub.4 or HCl containing Fe ions.

The first character consists of the recovery of Fe hydroxide and Cl.sup.- ions in which Fe.sup.2+ ions in one part of catholyte drawn off are oxidized to Fe.sup.3+ ions by contact of air or oxygen, iron hydroxides produced in the above process are filtered off and the filtrate is reused for the cathode compartment or strip solution in the third step.

The second character consists of the recovery of high-purity electrolytic iron deposited on the cathode and Cl.sup.- ions by transferring them into the central compartment.

(15) A modification of this invention comprises the following steps:

The first step in which a waste HCl containing high total Cl.sup.- ions concentration and abundant Fe ions is introduced into the anode compartment divided with diaphragm, an oxidation reduction cell is constructed by connecting the electrodes set in the anode compartment and the cathode compartment in which a solution containing a large amounts of Fe.sup.3+ ions passes with wire and Fe.sup.2+ ions in the solution introduced into the anode compartment are oxidized to Fe.sup.3+ ions.

(16) The second step in which Fe.sup.3+ ions in the resultant aqueous solution from the first step are extracted into the organic phase as ferric chloride complex with contact of an organic solvent (C) containing a phosphoric acid ester or a primary, secondary, tertiary or quarternary amine.

(17) The raffinate from the second step is introduced into the central compartment in the diaphragm-electro-bath for Fe recovery in the third step and hydrochloric acid is recovered by Cl.sup.- and H.sup.+ ions transferred through the diaphragms and recycled to the reusing process.

(18) The organic solvent (C) extracted ferric chloride complex in the second step is stripped with contact of water or one part of catholyte in the electrolysis process for Fe recovery in the third step and regenerated.

(19) The said strip solution containing Fe.sup.3+ ions is introduced into the cathode compartment in the first step, an oxidation-reduction cell is constructed between the said strip containing Fe.sup.2+ ions solution in the cathode compartment and the waste acid in the anode compartment divided with diaphragm by connecting cathode and anode with wire and the Fe.sup.3+ ions in the said strip solution are converted to Fe.sup.2+ ions.

(20) An outer cell can be connected to promote the transfer of free acid produced by the reduction of Fe.sup.3+ ions into the anode compartment through the diaphragm.

(21) The aqueous solution containing the Fe.sup.2+ ions by being reduced is fed into the cathode compartment in electro-bath for Fe recovery in the third step. High-purity electrolytic iron is obtained in proceeding of electrolysis in the cathode compartment and Cl.sup.- ions produced at the same time are recovered by transferring them into the central compartment through the diaphragm.

(22) Fe.sup.2+ ions in one part of catholyte taken off are oxidized to Fe.sup.3+ ions with contact of air or oxygen. Fe hydroxide produced in the above process is filtered off and the resultant filtrate is recycled to the cathode compartment or reused for the strip solution in the second step.

As described above, this invention offers the recovery method of acid containing Fe ions with the character which high-purity electrolytic iron or Fe hydroxide is obtained and reusable HCl or H.sub.2 SO.sub.4 is recovered at the same time.

The following advantages may be obtained in practice of this invention.

(1) The choice of apparatus materials is very easy by means of the operation of whole apparatus at room temperature and even heating in the pickling process is unnecessary.

(2) In use in the pickling process the HCl loss as its fume is diminished by capable lowering total Cl.sup.- ions concentration in the aqueous solution and consequently the inferiority of environment can be prevented.

(3) The enhancement of economical value based on capable closed-circuit, non-pollution and production of additional valuable high-purity electrolytic iron and iron hydroxide.

(4) The low labor cost based on easy operation control of apparatus and capable automatic operation.

(5) The low maintenance cost based on rare accident by means of few movable parts of apparatus and high operating rate of apparatus.

(6) Little limitation of apparatus scale, capable manufacture of equipments from large capacity to small capacity and easy stepwise development.

The present invention will now be described, through only by way of illustration, in the following Example which shows preferred aspects of the invention, by reference to the accompanying drawings.

FIG. 1 shows a flow-sheet of the reusable HCl recovery from waste hydrochloric acid containing a relative little content of free acid;

FIG. 2 shows a flow-sheet of the reusable H.sub.2 SO.sub.4 recovery from waste sulphuric acid;

FIG. 3 shows a flow-sheet of the reusable HCl recovery from waste hydrochloric acid containing a large amounts of free acid and total Cl.sup.- ion;

FIG. 4 is a graph showing the oxidation-reduction process in the diaphragm-electrolysis process;

FIG. 5 is a graph showing the extraction equilibrium curve of Fe.sup.3+ ions in HCl solution with D2EHPA;

FIG. 6 is a graph showing the relation between stripping coefficient of Fe.sup.3+ ions extracted with D2EHPA and HCl concentration;

FIG. 7 is a graph showing the back-extraction equilibrium curve of Fe.sup.3+ ions extracted with D2EHPA;

FIG. 8 is a graph showing the HFeCl.sub.4 extraction equilibrium curve with TBP (Tri-butyl phosphate) or TOP (Tri-octyl phosphate);

FIG. 9 is a graph showing the relation between back-extraction of HFeCl.sub.4 in the organic phase and Cl.sup.- ions concentration;

FIG. 10 is a graph showing the electro-bath for iron recovery (3 baths);

FIG. 11 is a graph showing the electro-bath for iron recovery (4 baths);

FIG. 12 is a graph showing the relation between transferred amounts of Cl.sup.- ions in the iron electrolysis process and difference of total Cl.sup.- ions concentration;

FIG. 13 is a graph showing the relation between amounts of free acid in the catholyte and iron electrodeposition efficiency;

FIG. 14 is a graph showing the relation between pH value of catholyte and iron electrodeposition efficiency; and

FIG. 15 is a graph showing the electro-bath for iron recovery (4 baths-2 anion exchange membrane).

The following example is based on the experiments carried out by the inventors. The typical chemical analysis of waste hydrochloric acid used for test is showed as follows:

(1) The First Stage--Oxidation and Reduction

The anode compartment divided with diaphragm is continuously fed with the waste acids and while the cathode compartment is fed with the synthesized HCl solution containing Fe.sup.3+ ions (see FIG. 4). The used diaphragms consists of tetra-fluo ethylene, polyethylene acetic cellulose, diffusion-dialysis (SELEMION-DMV, trade name, produced by Asahi Glass), cation exchange (SELEMION-CMV, trade name, produced by Asahi Glass) and anion exchange membrane (SELEMION-AMV, trade name, produced by Asahi Glass). The analogous diaphragms, e.g., PVC (poly vinyl chloride) membrane, ACIPLEX-A101 (trade name, produced by Asahi Kasei), ACIPLEX-K101 (trade name, produced by Asahi Kasei), NEPTON (trade name, produced by IONICS Corp.) and NAFION (trade name, produced by Dupont Corp.), etc, can be used.

The flow path of catholyte and anolyte in the apparatus is connected with countercurrent flow through the diaphragms and the oxidation-reduction in the each final bath of both compartments is promoted by outer cell. The used anode comprises carbon, resin-impregnated carbon, Pt-plating Ti, Ta, Nb, Rh or metal oxide-coating materials and the shape of plate, net or expanded metal can be used. The used cathode is stainless steel and the shape of its plate is conventional for the operation after iron recovery.

(2) The second stage--The Extraction of Fe.sup.3+ ions

The Fe.sup.3+ ions in the waste acids from the first stage are extracted into the organic phase with contact of the organic solvent (A) containing a di-alkyl phosphoric acid, such as, for example, D2EHPA (Di-2-ethyl hexyl phosphoric acid) or HDBP (Monododecyl phosphoric acid) (see, FIG. 5). The organic solvent (A) may be a mixed solvent D2EHPA and H.sup.+ -exchangeable solvent, such as, naphthenic acid or oxime group. The raffinate in which the Fe.sup.3+ ion were extracted off is recycled to the using parts. The Fe.sup.3+ ions extracted into the organic solvent (A) are stripped from the organic phase with contact of a solution containing over 80 g/l of HCl and the organic solvent (A) is regenerated as the following expression.

(see FIGS. 6 and 7)

(3) The Third Stage:

The enhancement of Fe.sup.3+ ions concentration in the HCl-strip solution from the second stage is difficult as shown in FIG. 7. Moreover, since a solution of high free acid concentration is required for stripping the Fe.sup.3+ ions from the organic solvent (A) as shown in FIG. 6, the Fe.sup.3+ ions are unable to be stripped with water or the catholyte in the fourth stage.

An organic solvent (C) containing a phosphoric acid ester, such as, TBP, TOP, DBBP (di-butyl butyl phosphonate) or TOPO (tri octyl phosphine oxide) is selected in view of water balance in the electrolysis process and capable strip with the catholyte. Moreover, an organic solvent (C) containing a primary, secondary, tertiary or quarternary amine may be used. A further possibility is to employ as organic solvent (C) a mixture of a phosphoric acid ester and an amine. Examples of suitable amines include Primene-JMT (trade name, primary amine produced by Rohm and Haas), LA-1 (trade name, secondary amine produced by Rohm and Haas), Alamine 336 (trade name, tertiary amine produced by General Mills) and Aliquat 336 (trade name, quarternary amine produced by General Mills). The analogous phosphoric acids or amines may be used.

The iron species are extracted as ferric-chloride complex as shown in the following reactions.

(see FIG. 8)

As described above, the total Cl.sup.- ions concentration in the raffinate is decreased owing to the extraction of Fe.sup.3+ ions as ferric chloride complex and consequently the raffinate can not be used for stripping in the second stage. Therefore, if necessary, the raffinate is treated by diffusion-dialysis process and introduced into the central compartment in the electro-bath for iron recovery in the fourth stage. Thus, the HCl concentration in the raffinate is increased by H.sup.+ and Cl.sup.- ions obtained through the diaphragms and can be used for stripping the organic solvent (B). The ferric chloride complex extracted into the organic solvent (B) is stripped from the organic phase with contact of water or catholyte in the fourth stage. FIG. 9 shows the relation between total Cl.sup.- ions concentration in the strip solution and strip efficiency of ferric chloride complex from the organic solvent (B).

Since it is found from FIG. 9 that the catholyte in the fourth stage containing 250-300 g/l of total Cl.sup.- ions is able to strip the ferric chloride complex from the organic solvent (B) by increase of stripping stage, Fe ions can be supplied into the catholyte for iron electrolysis. However, the amount of Cl.sup.- ions transferred from the organic solvent (A) with Fe ions is 4 moles per 1 mole of Fe. While, the amount of free Cl.sup.- ions, produced by reduction of Fe.sup.3+ ions in the oxidation-reduction cell which is constructed between the strip solution introduced into the cathode compartment in the first stage and the solution in the anode compartment contains Fe.sup.2+ ions is 2 moles per 1 mole of Fe.

Therefore, in prior of the iron electrolysis process in the fourth stage the amount of free acid is lowered by the first stage and dialysis process and consequently the inferior efficiency of iron electrode position may be prevented with the increase of free acid as shown in FIG. 13. In the dialysis compartment Cl.sup.- ions or HCl can be transferred from the solution containing higher total Cl.sup.- ions to the one containing lower total Cl.sup.- ions using SELEMION-DMV. Both diffusion-dialysis and electro-dialysis can be adopted in the dialysis compartment.

(4) The Fourth Stage--Electrolysis for Fe Recovery

The solution from the cathode compartment in the first stage in which Fe.sup.3+ ions are reduced to Fe.sup.2+ ions is mixed with the solution circulating in the cathode compartment in the electro-bath for Fe recovery. As described above, when the solution from the first stage contains abundant free hydrochloric acid, if necessary, it is mixed with the catholyte after it passed through the diffusion-dialysis or electro-dialysis compartment.

The summary of electro-bath for iron recovery is shown in FIGS. 10 and 11. 3-baths and 4-baths types were tested and the latter is favourable for the high concentration of HCl in the central compartment.

Fe ions can be recovered as iron hydroxide besides metallic iron. FeOOH produced in the oxidation of Fe.sup.2+ ions to Fe.sup.3+ ions with contact of air or oxygen is separated. Moreover, the Fe.sup.3+ ions are reduced to Fe.sup.2+ ions in the cathode compartment and free Cl.sup.- ions are transferred into the central compartment. Fe ions can be removed from the system as hydroxide form by repeating the above treatment.

The cation exchange membrane is required for the membrane between anode and central compartments in electro-bath. ACIPLEX-K101 and SELEMION-CMV were used for the test. The similar membranes having a function of selective permeability of cation, such as, NEPTON and NAFION, etc., may be used except the above membranes.

The membrane used between anode and central compartments is not necessarily an anion exchange membrane. As shown in FIG. 12, the membranes except the anion exchange membrane are superior in the transfer rate of Cl.sup.- ions produced in the electrolysis from the cathode compartment to the central compartment.

SELEMION-AMV and ACIPLEX-A101 as an anion exchange membrane were tested. SELEMION-DMV was used as a diffusion-dialysis membrane. The used fine pore diaphragms consist of tetra fluo-ethylene, acetic cellulose, polyethylene, polypropylene and poly vinyl chloride membranes. The diaphragm divided the central compartment in electrolysis of 4- baths was a fine pore diaphragm or cation exchange membrane.

The anode used is lead, Ti-Pt, Ti-Rh, Ta or Nb. The electrodes having a function of catalysis and coated with metal oxide may be used. The cathode used is steel, stainless steel or Ni plate and the shape of plate is usually used in view of after-treatment of electrolytic iron. The net type may be used. The supporting electrolyte added into the catholyte is CaCl.sub.2, NH.sub.4 Cl, (NH.sub.4).sub.2 SO.sub.4 or a mixed electrolyte of the above one but CaCl.sub.2, NH.sub.4 Cl or a mixture of CaCl.sub.2 +NH.sub.4 Cl is often used in regard of the third stage. The results of electrolysis test are shown in the following table 2.

The chemical analysis of waste acid discharged in the production of TiO.sub.2 is shown in the following table and the above waste acid was used for the test.

The waste acid contains a little amount of Al, V, Mn and Mg ions besides the said chemical species.

(1) The First Stage--The Oxidation of Fe.sup.2+ ions

The waste acid is introduced into the anode compartment due to the oxidation and the HCl solution containing Fe.sup.3+ ions is introduced into the cathode compartment divided with diaphragm. The oxidation-reduction cell is constructed by connecting cathode and anode set in the both compartments with wire. Fe.sup.2+ ions in the anode compartment are oxidized to Fe.sup.3+ ions and Fe.sup.3+ ions in the cathode compartment are reduced to Fe.sup.2+ ions.

The used diaphragm is the cation exchange membrane in order to prevent Cl.sup.- ions. SELEMION-CMV and ACIPLEX-K101 were used for the test but the analogous cation exchange membrane, such as, NEPTON and NAFION, etc., may be used.

In the above test, the each final compartment of anode and cathode compartments are divided with the diaphragms and connected to an outer cell. The analogous membranes having a function of selective permeability of cation besides the above membranes may be utilized. Furthermore, the use together with a fine pore membrane is able to prevent the transfer of anion and the degradation of cation exchange membrane by Cl.sub.2 gas.

(2) The Second Stage--The Extraction of Fe.sup.3+ ions

The Fe.sup.3+ ions contained in the solution from the anode compartment in the first stage are extracted into the organic phase with contact of an organic solvent (A) containing a di-alkyl phosphoric acid, such as, D2EHPA or HDBP.

The resultant Fe.sup.3+ ion free aqueous solution is regenerated and recycled to the using part. However, in the production of TiO.sub.2 the H.sub.2 SO.sub.4 concentration in the resultant solution is concentrated to 70-90% by the concentration process and reused to dissolve raw materials. The Fe.sup.3+ ions extracted into the organic solvent (A) are stripped from the organic phase by contact with HCl and then the Ti ions in the organic phase can be stripped by contact with (NH.sub.4).sub.2 CO.sub.3 +NH.sub.3 solution.

Since the organic solvent (A) after the stripping of Ti ions is converted to the NH.sub.4 -type, it is converted back to the H-type by contact with H.sub.2 SO.sub.4 and reused to extract the Fe.sup.3+ ions. When the Ti ions are not present, the Fe.sup.3+ ions are stripped by contact with HCl and reused to extract the Fe.sup.3+ ions after washing with water or 1-5% of H.sub.2 SO.sub.4 solution.

The third and fourth stages are omitted due to the same as those of the example 1.

The waste acid discharged in the production of steel wire, such as, piano wire, by lead patenting method was used for the test. Its chemical composition is shown as follows.

(1) The First Stage--The Oxidation of Fe.sup.2+ ions

The above solution is introduced into the anode compartment divided with diaphragm and the cathode compartment is filled with a solution containing Fe.sup.3+ ions. The oxidation-reduction cell is constructed by connecting a carbon or metal electrode set in both compartments. The Fe.sup.2+ ions in the anode compartment are oxidized to Fe.sup.3+ ions, while the Fe.sup.3+ ions in the cathode compartment are reduced to Fe.sup.2+ ions.

An outer cell is connected in order to promote the reduction of Fe.sup.3+ ions in the cathode compartment to Fe.sup.2+ ions and accomplish the oxidation of Fe.sup.2+ ions by continuous countercurrent. The rate of conversion is enhanced and the free HCl in the catholyte is transferred into the anode compartment.

Ti-Pt, Ti-Rh and Ta electrodes besides carbon electrode may be used as anode. Mild steel and Ni electrodes can be used as cathode. The shapes of used electrode are plate, net and expand.

The used diaphragm is an anion exchange membrane, such as, SELEMION-AMV, NEPTON or an ordinary fine pore diaphragm besides a diffusion-dialysis membrane. Tetra fluo-ethylene, poly vinyl chloride, polyethylene or acetic cellulose membranes was used for the test. The analogous diaphragms may be used.

This example is different from examples 1 or 2. In the present example, the best diaphragm is a diffusion-dialysis membrane, such as, SELEMION-DMV with the object of transferring HCl, present at the same time of the reduction of Fe.sup.3+ ions in the first stage, into the anode compartment. The analogous dialysis membranes having the same function may be used.

It is found from FIG. 13 that the electro-deposition efficiency of iron favourably increases as the concentration of free HCl in the solution from the cathode compartment decreases.

(2) The Second Stage--The Extraction of Fe.sup.3+ ions ferric Chloride Complex

The total Cl.sup.- ions concentration in the waste acid in which Fe.sup.2+ ions are converted to Fe.sup.3+ ions in the first stage is raised by Cl.sup.- ions and HCl transferred from the cathode compartment. The Fe.sup.3+ ions in the resultant waste acid are extracted as ferric chloride complex into the organic phase with contact of an organic solvent (C), such as, a phosphoric acid ester or a mixed solvent of amines selected from a primary, secondary, tertiary or quarternary amine group.

TBP, TOP, DBBP or TOPO as a phosphoric acid ester was used. Primene JMT (a primary amine), LA-1 (a secondary amine), Alamine 336 (a tertiary amine) or Aliquat 336 (a quarternary amine) as an amine was used. The analogous phosphoric acid esters or amines may be used.

The raffinate from the second stage has the following chemical composition.

The ferric chloride complex extracted into the organic solvent (C) is stripped from the organic phase with contact of water or the catholyte in the fourth stage and the organic solvent (C) is regenerated. The said strip solution in which the Fe.sup.3+ ions were increased is supplied to the cathode compartment in the first stage. The oxidation-reduction cell is constructed between the above solution in the cathode compartment and the waste acid containing Fe.sup.2+ ions in the anode compartment divided with diaphragm. After the reduction of Fe.sup.3+ ions to Fe.sup.2+ ions, the resultant solution is mixed with the catholyte circulating in the electro-bath for iron recovery in the third stage.

(3) The Third Stage

In the case of increasing the concentration of recovered HCl in the central compartment of electro-bath for iron recovery it is unable to completely prevent that the Cl.sup.- ions mix to the anode compartment by means of the selective character of cation exchange membrane used in the anode compartment side. Therefore, the used anode materials are limited.

As shown in FIG. 15, it was capable to prevent that the Cl.sup.- ions enter to the anode compartment by enhancement of number of compartment using cation exchange or diffusion-dialysis membrane.

The catholyte containing 100 g/l of Fe.sup.2+, 40 g/l of Ca and 195 g/l of total Cl.sup.- and having pH value of 3.8 is drawn off and oxidized for 6 hrs. with O.sub.2 gas at 60.degree. C.

The chemical composition of filtrate after aging is shown as follows.

The said solution is recycled to strip the ferric chloride complex extracted in the organic solvent (C) or mixed with the catholyte in electro-bath for iron recovery to reduce Fe.sup.3+ ions and transferred free acid produced in the reduction process to the central compartment through diaphragm. In repeating the above operation, Fe ions can be fixed in the form of hydroxide without the reduction until metallic iron. The iron hydroxide obtained in this process is high-purity and may be used for ferrite or pigment by further purification.