PURIFICATION PROCESS FOR SPENT PICKLING BATHS

Abstract: Purification process for spent, preferably hydrochloric acid based pickling baths, which have been used for pickling of steel articles in a hot dip galvanization process. Metal ions present in the pickling bath in the form of iron chloride complexes or zinc chloride complexes are brought in contact with an anion adsorbing material, preferably a conventional chloride ion-saturated anion exchanger, whereby the zinc chloride complex is selectively adsorbed. The solution containing the iron chloride complex can be used directly for the preparation of water purification chemicals. The ion exchanger is eluted or washed with water, whereby the zinc chloride complexes disintegrate, hydrochloric acid and zinc chloride are obtained in the washing solution, at the same time as the ion exchanger is regenerated while maintaining the same activity. The washing solution can, by a multistep washing in counter-current, be given a high enough concentration of zinc chloride so that it can be recirculated without further treatment to the flux bath in the hot dip galvanization process. If the chloride content of the spent pickling bath is not sufficient sot hat all metal ions will be present in complex form, this can be adjusted simply by addition of hydrochloric acid.

Description: Purification process for spent pickling baths The present invention relates to a purification process for spent pickling baths , preferably based on hydrochloric acid, which have been used for pickling steel articles. In hot dip galvanizing of steel articles, the .steel must be pretreated if the zinc is to adhere. This pretreat- ment can for example include the steps of degreasing, pickling, washing, neutralization, flux bath treatment, and thereafter immersion in molten zinc. Pickling consists of dipping into a pickling bath which is usually hydro¬ chloric acid, but which also can be sulphuric acid. The flux bath has a high concentration of zinc chloride and ammonium chloride. When pickling steel articles, the normal starting concentration of hydrochloric acid in the pickling bath is about 10-15 %. During pickling the bath is contaminated with among other things iron ions and the pickling bath can be said to be spent when its concentra¬ tion of Fe is about 80-120 g/1 and when the hydrochloric acid concentration has gone down to about 5-10 %. The pickling bath is also unavoidably contaminated with zinc, which comes primarily from unsuccessful galvanizations where the zinc must be removed by pickling before a new galvanization can be attempted (so-called burning-off pickling bath) . The pickling bath- is also contaminated by the zinc cloud surrounding the hot dip galvanizing process and from the hooks which convey the articles past the various stations. The amount of zinc in a spent pickling bath can be about 1-20 g/1 Zn. in modern day chemical processes it is especially important that the waste solutions produced be able to be reused, encapsuled, incinerated or otherwise prevented from affecting the external invironment. As far as we know there is at present no industrial process in operation for purifying spent pickling baths. Conventional methods of separation between iron and zinc in this type of solution, e.g. by fractionated neutraliza¬ tion at gradually increasing pH or by adsorption on metal iron adsorbing material, e.g. ion exchanger, provide a poor separation effect, involve relatively high costs and give rise to residual waste problems in the form of sludge. Another process has been developed lately for purifica- tion of pickling baths, namely the so-called MX method. In principle it involves the transfer of the metal ions by the formation of complexes into a kerosene phase, which is then extracted with water. This method is, however, quite 3 costly and a major disadvantage is that 1 m pickling bath 3 gives rise to about 2 m waste liquid which must then be taken care of. Thus the purpose of the present invention is to achieve a simple and inexpensive process for purification of spent, preferably hydrochloric acid based, pickling baths, which gives rise to residual material which can be reused in the process and/or used for other purposes, and which is independent of the amounts of metal ions contaminating the pickling bath. The method according to the invention is characterized in that metal ions present in the spent pickling bath in the form of anion complexes are brought in contact with an anion-adsorbing material, which selectively adsorbs one of the metal ion complexes, whereafter the adsorbed metal ion complex is washed out during the formation of a concentrate solution which can be used without further treatment. The spent pickling bath is thus primarily contaminated with iron- and zinc ions, and the method according to the invention is primarily directed to the separation of the same. Depending on the hydrochloric acid content of the pickling bath and the content of contaminating metal ions, the metal ions are present more or less in the form of chloride complexes. As was mentioned in the introduction, normal contents in a spent pickling bath are 1-20 g/1 Zn an 80-120 g/1 Fe. In order for all of the zinc- and iron ions to be present as metal ion chloride complexes, the chloride content of the pickling bath must be about 100-500 g/1 and the hydrochloric acid concentration be about 1-5 ekv. OMPI IPO In certain cases, however, the zinc concentration can be very low, e.g. 0.5 g/1 or even 0.1 g/1. Even at these low concentrations, the invention works satisfactorily and from a content of about 0.1 g/1 Zn prior to purification, a content on the order of 0.001 g/1 zinc can be obtained after purification. When the pickling bath has functioned as a burning-off bath after unsuccessful galvanizations, the zinc content in the bath is instead quite high, for 'example about 80 g/1 or sometimes even higher, at the same time as the iron content can be very low, for example, about 20-30 g/1- Even in these cases the process according to the invention works just as satisfactorily, as it does in those cases where the iron content reaches up to 150 g/1 or higher. If the chloride content of the spent pickling bath is not sufficient for th.e complex formation, the chloride content can be adjusted quite easily by the simple addition of HC1. By virtue of the fact that the metal ions are present in the form of anion complexes, they can be separated with the aid of an anion adsorbing material, which provides a substantially better selectivity than with the previously mentioned separation by means of cation exchangers. Examples of anion adsorbing materials which can be used are activated charcoal, molecular sieves and the like. Conventional anion exchanger materials, such as Amberlite IRA-400, have proved themselves to be especially well suited. Under prevailing conditions, the zinc chloride complex is adsorbed selectively, while the iron chloride complex follows with the solution, when the pickling bath is brought in contact with said anion exchanger material. The residual solution which passes through the ion exchanger and which contains the iron chloride complex can be used directly for the preparation of water purification chemicals for example, which is one of the great advantages of the invention. But, of course any suitable anion adsorbing material can be used which selectively adsorbs the zinc chloride complex. In order for an ion exchanger process to be usable in industrial processes, however, the ion exchanger must be able to be regenerated to the same active levels without costly procedures or materials. This is possible with the process according to the invention. The ion exchanger is eluted-or--washed with wate The ion concentration is thus diluted and the zinc chlorid complexes disintegrate. The wash water carries with it zin chloride and hydrochloric acid at the same time as the ion exchanger is regenerated and again contains chloride ions. The ion exchanger is thereby completely restored wit the same activity and mere water.is used as an elution agent, a very inexpensive raw material. As was mentioned previously, the hot dip galvanization process comprises a flux step, in which the flux bath has a high concentration of zinc chloride and ammonium chlorid Now if the zinc chloride obtained in the regeneration of the ion exchanger can be obtained in sufficiently high concentration, the washing liquid can be recirculated directly back to the hot dip galvanizing process. According to the invention this. is achieved by carryin out the washing in steps countercurrent to the normal flow through the ion exchanger. Only limited amounts of water are used, which are kept separate and used so that the volume with the highest degree of contamination washes first, whereby the contaminant content is additionally increased. Thus in each regeneration cycle only a small amount of washing liquid with high zinc content is removed and a small amount of pure water is added. This results in about 10-100 liters washing liquid having a high enough zinc chloride content that it can be directly added to the flux bath, per 1 m spent pickling bath. That this is a major step forward is revealed for example by a comparis 3 with the above mentioned MX method, where 2 m of waste liquid were obtained per 1 m pickling bath, and where thi waste liquid then required further treatment. No portion of the pickling bath is wasted, since ' one portion can be recirculated in the process and the other portion can be OMPI used for the production of water purification chemicals, for example. To illuminate the invention further, several examples are described below, carried out on a laboratory scale. Example 1. 100 ml pickling bath was allowed to flow through a bed containing 20 ml anion exchanger substance. The composition of the pickling bath was 147 g/1 Fe, whereof 6 g/1 was Fe , 2.64 g/1 Zn and 42 g/1 HCl. The zinc content in the treated pickling bath, i.e. that which has flowed through the substance, rose during the test from 0.75 mg/1 after 20 ml to 195 mg/1 after 100 ml. The bed was then washed with 100 ml pure water which was allowed to run-through the bed. The zinc content in the washing liquid was after 40 ml 50 mg/1 and after 60 ml, 4700 mg/1. 93 % of the added zinc was washed out. Thereafter 100 ml pickling bath was allowed to flow through the bed. The zinc content was after 100 ml, 225 mg/1. The bed was then washed with the previous washing water. The highest content of zinc was 500 mg/1. The bed was washed with 100 ml pure water and the highest content of zinc was then 7700 mg/1. Example 2. 30 liters of pickling bath were supplied to 2.3 liters anion exchanger substance. The composition of the pickling bath was 47 g/1 Fe, whereof 1.4 g/1 Fe + , 3.8 g/1 Zn and 111 g/1 HCl. The zinc content in the treated pickling bath was after 30 liters, 25 mg/1. The bed was then regenerated with 15 liters of water. The zinc content after 2 liters was 15 g/1 and after 13 liters 570 mg/1. 62 % of the added zinc was washed out of the bed. Example 3. 20 liters of pickling bath with the composition 41 g/1 Fe, 7.3 g/1 Zn and 14 g/1 HCl were supplied to the same anion exchanger bed as in Example 2. The zinc content in the treated pickling bath was after 10 liters 0.7 g/1 and after 20 liters 4.0 g/1. The bed was then regenerated with washing liquid according to the following Table. Table Amount of washing liquid Zinc content liters in out 3 15 22 5 14 17 5 9.5 14 4 0 11 By division of the washing liquid into portions, ther was obtained from 20 liters of pickling bath with 7.3 g/1 Zn, 20 liters purified .pickling bath with 1.6 g/1 Zn and 6 liters washing liquid with 19 g/1 Zn. Thus the same amo of zinc as was added to the bed could be removed with the washing liquid. The invention is not limited to hydrochloric acid base pickling baths, but can also be used for sulphuric acid based pickling baths. In that case, however, the entire amount of chloride required for transferring the metal ion to anion complexes must be added. The invention is not limited solely to the separation of iron- and zinc ions. The pickling bath becomes contaminated with lead for example when wire is to be galvanized, due to the fact that when wire is drawn it is often passed through a bath of molten lead to lubricate the " same. Drops of lead adhering to the wire are then pickled off the wire in the pickling bath. Lead also forms chloride complexes, and through properly balanced conditio can be separated from the iron in the pickling bath. The invention should not be regarded as being limited solely to purification of spent pickling baths either. It can of course be used in any situation requiring the separation of metal ions from each other in hydrochloric acid solution. 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Claims: CLAIMS 1. Method of purifying spent pickling baths, preferably based on hydrochloric acid, which have been used for pickling steel articles, characterized in that the pickling bath, with metal ions therein in the form of anion complexes, is brought in contact with an anio adsorbing material, which selectively adsorbs one of the metal ion complexes, followed by simultaneous washing and regeneration of the anion adsorbing material. 2. Method according to Claim 1, characterized in that iron ions and zinc ions are present as iron chloride complexes and zinc chloride complexes, respectively, in the pickling bath and that the zinc chloride complex is selectively adsorbed on to the anion adsorbing material. 3. Method according to Claim 1, characterized in that the anion adsorbing material is a conventional chloride ion-saturated anion exchanger. 4. Method according to Claim 3, characterized in that the anion -exchanger is simultaneously both regenerated and washed with water, whereby the zinc chloride complex decomposes, zinc chloride and hydrochloric acid are formed and are carried with the wash water, and that the anion exchanger is thereby regenerated so as to preserve its activity. 5. Method according to Claim 4, characterized in that the washing takes .place in countercurrent to the normal flow and in several steps and that the-same washing solution is used several times to achieve a high concentration in the residual solution. 6. Method according to Claim 5, characterized in that the washing is carried out in 2-5 steps with separate volumes of water and that the wash volumes for each cycle are used one step earlier in the washing. 7. Method according to Claim 2, characterized in that the residue solution containing only iron ions is used for preparing water purification chemicals. 8. Method according- to Claims 5 and 6, characterized in that the washing solution is used for the preparation of fluxing agent for a hot dip galvanization process

Inventors: RENNERFELT L
Applicants: RENNERFELT L
Application year: 1980
Main classification: C23G1
IPC codes: B01J41, C23C2, C23G1