DK145682B - PROCEDURE FOR PREPARING PREPARING SOLUTIONS USED FOR IRON PREPARATION - Google Patents

Description

145682

The invention relates to a process for processing pickling solutions used in connection with pickling of iron and containing not more than 100 g of sulfuric acid per liter. liter and at least 25 g of iron ions per liter. per liter by a combination of electrolytic cleavage of the ferrous sulfate and electrodialytic isolation of acid and sulfate salt components in the used pickling solution.

By removal by sulfuric acid of iron oxides from semi-finished products in the iron industry, such as iron sulphate is formed from iron plates, iron bands, iron wire or profile iron which can be removed by cooling the solution 10 or by vacuum crystallisation from the used pickling solution in the form of iron sulphate heptahydrate (FeSO4 .THgO). As the speed of the pickling process decreases with the declining sulfuric acid content of the used pickling solution, the used pickling solution always (even in its securely "fully worked" state) still contains unused acid 15 in an amount of at least 20-30 g of sulfuric acid per liter. However, acidity after pickling depends on the process by which pickling is carried out and can be up to or even exceeding 100 g per liter. In order to increase pickling speed, in practice, an effort is usually made to increase the acid concentration, which is why a full 20 utilization of the sulfuric acid, ie a so-called depletion of the bath, is avoided.The productivity of a pickling plant increases proportionally with the pickling speed.

However, there is a limit to how far these efforts can go, as the acid loss and, consequently, the cost of pickling 25 grows proportionally with the increased acid content of the pickling solution, i.e. the amount of acid added to the pickle to be removed from the pickle unused. For the same reason, environmental pollution problems are becoming increasingly prevalent, as discharges of the acidic used pickling solutions are strictly prohibited according to those for watercourses, etc. 30 applicable rules. If, as is often the case in practice, the used pickle solution is neutralized with lime, the cost and difficulty of this measure also increase proportionally to the acidity of the used pickle solution. A further disadvantage of the treatment with lime is that the neutralization reaction is heterogeneous, so that even with an excess of lime there is no guarantee of complete neutralization of the acid. In addition, handling, transport and storage of the lime sludge is cumbersome and expensive. These work steps do not produce any useful results, e.g. in the form of recycling of chemicals.

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The method known in the industry for working up sulfuric acid used pickling solutions consists in the separation of the heptahydrate by vacuum crystal I isation. The acid concentration of the used pickling solution is increased because of the release of water to form crystal water and 5 vacuum vapor. Thus, the solution can be used again for pickling. However, a more widespread use of this process is ruled out as the heptahydrate, due to lack of demand, cannot be economically exploited, so most of this salt is brought to the landfill so that this method does not solve the environmental pollution problem either.

Recognition of these conditions has led research in the last few decades to work more on developing technical solutions that not only enable the recovery of the used stain solution's acid content, but also the conversion of the ferrous sulfate to sulfur-15 acid, which can be recycled. The electrochemical decomposition of ferrous sulfate in aqueous solution proceeding according to the reaction equation:

FeSO4 + H2O = H2SO4 + 0.5 O2 + Fe can serve as a basis for such solutions. In this reaction, an amount of 20 iron on the cathode is separated from the solution from the solution.

However, the realization of the electrochemical reaction stated in the equation is prevented by the fact that the iron can only be cathodically excreted from an acidic solution with poor efficiency. As the acid concentration increases, the iron secretion decreases and eventually stops completely, because hydrogen is formed at the cathode instead of a precipitate of iron.

In this way, instead of a breakdown of iron sulfate by electrolysis, eventually a decomposition of water takes place. Therefore, the solutions proposed so far are all aimed at limiting the decomposition of water.

The cathodic hydrogen production can e.g. is limited by the use of a liquid mercury electrode. At this cathode, the iron from an acidic solution can be precipitated in the form of an amalgam. Then, in a special room, the iron is anodically excreted from the amalgam. If an acid-free medium is used, the iron can be excreted even on a fixed electrode. This solution 35 is proposed by F. Aigner and G. Jangg under the title "Elektrolytische Auf-werkitung von verbrauchten schwefelsauren Beizlosungen" in Berg- und Hiittenmånnischen Monatsheften "(1969, pages 12-18). times, and the mercury electrode can only be used in horizontal placement and only with one side, the apparatus becomes disproportionately space-consuming and expensive. The high price of mercury increases the already considerable investment costs, and it is well known that the final product's manufacturing costs at all Methods operating with mercury cathodes are increased 5 as a result of significant losses of mercury.In the said publication, the authors themselves acknowledge that the economic value of the process is questionable and the specific energy requirements indicate those with the limit values of 12.5-13.5 kWh / kg Fe Similar objections may be raised to other methods based on the use of mercury cathodes. mmer i.a. expressed in A.T. Kuhn's publication 11A Review of the Role of Electrolysis in the Treatment of Iron Pickle Liquor "(Iron and Steel, June 1971, see 173-176). The said publication also otherwise provides valuable references for information on the state of the art, in which the article contains a list of the source material used 15 arranged according to the effects underlying the solutions in question, and the different methods are compared and evaluated in relation to each other.

Many researchers have also employed methods based on the use of permselective membranes. These methods are characterized in that the migration of hydroxonium cations (HgO +) to the cathode is prevented by a so-called ion exchange membrane placed between the anode and cathode compartment. However, the high electrical resistance of the membranes, their sensitivity to heat, acid and mechanical influences as well as their short service life and their high cost, despite promising results in laboratory studies, have prevented the use of methods based on such membranes to a greater extent. Against this background, besides the mentioned work by A.T. Kuhn also published "Treatment of Iron Containing i

Spent Sulfuric Acid by Electrolytic Dialysis "by the Japanese authors 30 Famurs and Ishio (Kogyo Kagaku Zseashi 69, 1435/1966) as well as the work" Separation of Iron Spent Sulfuric Acid by the Ion Exchange Resins "by the same authors (same place).

As a further possibility, there is also a method based on the use of bipolar active lead electrodes. In this process, in the aqueous iron sulphate solution during simultaneous cathodic iron precipitation, anodic lead sulphate is formed, which in a separate room is cathodically reduced to lead, thereby producing sulfuric acid. However, a prerequisite for the application of this method is that the iron sulfate solution introduced into the electrolysis cell is neutral.

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Therefore, this process cannot be used directly for reprocessing of acidic used pickling solutions, but only after a prior crystallization and subsequent resuspending of the heptahydrate (J. Ker-ti: Az active olomelek despite felhasznålåsi lehetosége as elektrokémiai 5 iparban (possibilities for using the active lead electrode in the electrochemical industry (MTA Kémiai CKzt. Kozl. 25, 251-281 / 1966 /, U.S. Patent No. 3,111,468 and English Patent No. 992,584).

The same condition (acid freedom) is applied to the composition of the used pickling solution by the method known from the specification of Hungarian Patent 10,156,806 (1967). In this process, the anode compartment separated by a diaphragm from the cathode compartment provides an excess amount of sulfate in excess of the sulfuric acid, which is preferably supplied in the form of ammonium or alkali metal sulfate. The excess sulfate ions retard the second dissociation step of the sulfuric acid so that the majority of the sulfuric acid protons are bound to SO₂ anions which, as a result of their negative charge anion formation, are prevented from penetrating the diaphragm and into the cathode compartment. In this, the used pickling solution is introduced, which, however, is only possible if the solution 20 is completely depleted of acid. If this condition is not met, as is also emphasized in the specification of said patent, the salt must be removed from the pickling solution by known methods (crystallization or thermal decomposition) and added to the catholyte.

An examination of the aforementioned problems has led to the recognition that an electrochemical cleavage of ferrous sulfate can be advantageously carried out in connection with an isolation of the content of the used pickling solution of sulphate salt and acid.

The invention thus relates to a process for working up pickling solutions used in connection with pickling of iron and containing not more than 100 g of sulfuric acid per liter. liter and at least 25 g of iron ions per liter. liters by a combination of electrolytic cleavage of the ferrous sulfate and electrodialytic isolation of acid and sulfate salt components in the spent pickle solution, which is characterized in that by adding salts an ammonium, magnesium or alkali metal sulfate concentration of 0.5-1 is obtained. 0 mol / l of the used pickling solution, whereupon the used pickling solution is introduced into the cathode compartment of an electrolysis apparatus composed of cells with anode and cathode compartments separated by diaphragms and flowed through the cathode compartment until the iron content of the solution is reduced to 5-15682 a value of 7-15 g Fe ++ / I, whereupon the solution is passed into the anode compartment of the apparatus and moved through it in the same direction as the flow direction maintained in the cathode space and the current density is maintained at a value of 15-22 A / dm calculated in relation to the area of the diaphragm, and 5 that the electrolysis is performed at a working temperature of 7Q-90 ° C.

By adhering to the above conditions, under continuous operation · under stationary equilibrium in the regeneration system, an equally favorable, stepwise concentration distribution for the acid, iron sulphate and bisulphate forming salts with a consequent repetitive, favorable acting transport process and electrochemical ionic swap chain process through the diaphragm. Due to the realized electrochemical ion exchange, it is not necessary to impose requirements on the diaphragm with respect to permselectivity.

The term "work-up" is understood to mean that the pickling solution can be taken out portion-wise from the pickling plant and continuously regenerated over an intermediate container.

Because, depending on the design of the pickling plant, it is not always possible to withdraw the used pickling solution continuously from the pickling plant, nor to return the regenerated continuously to the pickling plant and thereby create a closed circuit. In this case, the method according to the invention is conveniently carried out by inserting a larger intermediate storage container.

Thus, the crucial advantage of the process according to the invention is that a complete removal of the acid content of the used pickling solution or crystallization of the heptahydrate is not necessary for its implementation. This allows even in cases where the solution contains up to 100 g of F 2 SO 2 / liter (approximately 1 mol / liter) a direct and circulating regeneration of the pickling solution.

The effect of the electrochemical ion exchange processes and the step-wise concentration distribution mechanism is explained in greater detail below with reference to the drawing, which shows a plant for carrying out the method according to the invention. The acid concentration in the pickle solution flowing through the pickle containers P 2, 2, one at a time decreases from container to container as a result of the pickle reaction, while the iron concentration, also as a result of the pickle reaction, increases from container to container. The concentration of the bisulfate-forming addition salts (ammonium, magnesium or alkali metal sulfate) remains constant in the container row. The used stain solution is introduced at a constant rate into the cathode compartment of the electrolytic cell E ^, from which it flows through the cathode compartments of the cells Eg ··· One by one. The iron precipitates on the cathode. This cathode process is accompanied by dependence of the catholyte acid concentration of hydrogen formation. The remaining portion of the used pickling solution's acid content migrates in the form of HSO4 ions through the diaphragm into the anode compartment. At the same time, the cations of the added sulfates migrate from the anode compartment through the diaphragm into the cathode compartment.

Accordingly, the iron-and-acid concentration of the solution decreases from cell to cell during its flow through the cathode compartment, while the concentration of bisulfate-forming addition salts increases from cell to cell. The process can be summarized as follows: As a result of the ion transport, the acid in the pickle solution migrates through the diaphragm into the anode compartment (electrodialysis), and at the same time is replaced by the combined effect of the ion transport and the cathodic iron precipitate, the iron sulfate with ammonium, magnesium or alkali metal sulfate. The solution coming from the cathode compartment of the last cell (One), the so-called (final catalytic acid), is practically acid-free (pH = 1.6-2.0), its concentration of iron is 10 g Fe ++ / liter, and its concentration of sulfates are in the order of 1½ mol / liter.

The final catholyte is transported by means of a pump back to the beginning of the row of cells, while the transport and flow direction of the solution are otherwise obtained by incrementally lower placement of the cells and each cell as an outflow opening has an overflow. The pump transports the solution (final catholyte) into the anode compartment of cell E ^ f 25 from which it flows on through the anode compartments of cells Eg ... En-Below the acid content increases from cell to cell, while the concentration of the bisulfate-forming salts decreases from cell to cell. The incremental increase in the acid concentration is due partly to the anodic separation of the acid ions and partly to the ion transport already mentioned (the migration 30 of HSO4 ions). The decreasing salt concentration can be explained by the cations of the salts migrating through the diaphragm into the cathode compartment.

The acid content of the endanolite follows from the context: C = C. ·. + 1.75 (Fe ++ - Fe +!), Reg stain 'K max min' 35 where Creg is the final concentration of acid in the final oil listener in grams / liter,

Caffeine is the acid concentration of the used pickling solution in grams / liter, [[

Femax is the concentration of iron in the used pickling solution in grams / liter, and ++

Femin is the concentration of iron in the final catholyte in grams / liter.

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The factor of 1.75 is the ratio of the equivalent weights of the iron to the acid.

The bisulfate salt content of the final ethanol salt is necessarily exactly as high as the spent sulfur solution's bisulfate content, since the amount of addition salt that enters the electrolysis unit in the stationary state is exactly as large as the amount leaving it. According to the described mechanism, the electrochemical ion exchange is mediated by the bisulfate-forming salt, although the salt does not participate in the electrode process.

Compliance with the aforementioned process conditions is necessary because otherwise the described stepwise reduction of the concentrations in the cycle preceding the process cannot be provided or maintained and the electrochemical cleavage of the iron sulfate associated with the iron sulfate separation according to the mechanism described is not feasible. If the concentration of bisulfate-forming salt 15 in the pickling solution is less than 0.5 mole / liter, the amount of salt is insufficient to catalyze the electrochemical ion exchange chain. However, if the concentration is higher than 1.0 mol / liter, the pickling rate and the water solubility of the iron sulfate are visibly reduced. If the final acid content of the pickling solution is substantially higher than 100 grams / 20 liters, the efficiency of the stream decreases significantly and the specific energy demand for regeneration increases considerably. If no diaphragm was used, the anolyte and the catholyte would quickly mix with each other as a result of the gas formation and prevent the electrochemical ion exchange described. If all the iron were removed from the solution, the efficiency of the electrolysis would decrease and there would be a risk of the catholyte becoming alkaline. If the final catholyte per liter containing more than 15 g of iron ions, the di-valent iron ions would anodically oxidize to tri-valent ions, which would also result in a decrease in efficiency and also an increase in stain loss. If the flow directions in the cathode and anode compartments are not identical, the described ion exchange mechanism breaks down completely, and neither the electrodialytic isolation of the final acid nor the electrochemical cleavage of the iron sulfate can be carried out with satisfactory efficiency. At a current density of less than 15 A / dm, the apparatus is not sufficiently productive, and furthermore, the diffusion that occurs through the dia-fragment due to the ion transport will not sufficiently overcompensate, which would also result in a decreasing efficiency. At 2 a load with a current density of more than 22 A / dm, the structure of the coating on the cathode is not satisfactory and interferences occur. The transfer rate of the ions participating in the current transport depends on the temperature, and in the given system, the favorable transfer rates for the described ion exchange mechanism are obtained at temperatures above 70 ° C. At temperatures above 90 ° C, foam formation is increased undesirably, which also causes operating difficulties. Continuous operation is necessary because the setting of the stationary concentration distribution takes several hours and the circuit during this time cannot be maintained with sufficient efficiency.

Thus, interruption of the electrolysis is always associated with a significant loss of time and delay in production.

As for the regeneration, the iron concentration of the used pickling solution can be selected as desired. However, since the pickling rate decreases at an iron content of more than 90 g Fe ++ / Mter in the pickling solution, it is not appropriate to exceed this limit, and in particular not because at higher iron concentration there is also a greater risk of secretion of heptahydrate which causes operational disruption.

The flow rate of the solution is calculated and adjusted respectively according to the requirement for the stepwise concentration distribution, e.g. as follows: 20

v 1.83. Y. η. IN

v 100 (C-C...) Reg In the equation, V is the flow rate of the whole system in liters / hour, Y is the percentage efficiency, n is the number of series-connected electrolytic cells, and I is the strength of the electrolyzing current. in amps.

The other symbols have the same meaning as above. The factor 1.83 expresses the amount of the sulfuric acid produced at a degree of efficiency of 100%.

The question whether ammonium sulphate or other sulphate is used as bisulphate-forming additive or other sulphate has no effect on the efficiency, if only the molar salt of the additive salt in the pickling solution is the same in each case. However, the nature of the addition salt cation is of great importance for the structure of the cathodically precipitated iron.

2 If ammonium or magnesium sulfate or combinations of these two salts are used, respectively, a smooth cathode coating of leaf structure is obtained. Thus, if a cathode product which is to be worked up into blocks is to be recovered, the said addition salts are used.

In this case, the cathode plates, the thickness of which has emerged upon precipitation, are replaced from time to time by new plates (iron bases). The cathode plates removed from the solution are worked up by melting, e.g. for casting.

By using sodium or potassium sulphate, optionally a combination of these two salts, the cathodes in the cells located at the beginning of the cell line are formed in a coherent, not very glossy but rather matte coating. At the end of the cell row, where the iron content of the cathode compartment is lowered to less than 25 g Fe / liter and the molar salt of the addition salt is greater than 1, the iron is excreted in the form of 10 powders on the surface of the cathodes. In view of the high demand for iron powder, the production of this in itself is of great practical importance, and especially in the present cases where the extraction of iron powder is associated with the pickling process, utilization of the used pickling solution and the solution to an environmental pollution problem.

Thus, for the manufacture of iron powder, the process of the invention can be utilized in that sodium and / or potassium sulphate is used in the pickling solution in an amount of 0.5-1.0 mol / liter and in the cathode compartments as the bisulfate forming addition salt. the stationary iron concentration is less than 25 g Fe / liter, using aluminum, graphite or lead cathodes so that the iron powder can be easily removed from the electrodes which are occasionally lifted out of the solution by trimming or brushing. (Using iron electrodes, the precipitated particles would adhere more strongly to the plate).

If the entire iron content of the used pickling solution is to be recovered in the form of iron powder, a pickling technology must be used in which the iron content of the used pickling solution does not exceed a value of 25 g Fe ++ / liter and the acid content is less than 20 g / liter. These conditions can be met by open container systems.

An apparatus for carrying out the method according to the invention consists of incrementally arranged series connected cells having spaced apart electrode spaces. In the individual cells, the cathode and anode compartments alternate in succession, and between each of these a diaphragm is arranged. In the individual cells, bores are provided in the cell wall, connecting the cathode compartments with each other, as well as the anode compartments with each other. In this way, anolyte and catholyte can flow independently. In order to pass the solution from each cell's anode room or cathode room to the next cell's anode room or cathode room, each cell is provided with level guiding overflow. In the individual cells, the anodes and cathodes are electrically parallel to each other but independently connected to a busbar. In this way, it is possible independently for the other cathodes to lift any cathode out of the liquid. If iron powder is to be prepared in the manner already described, iron plates are used as cathodes at the beginning of the cell line and at the end of the cell line (more specifically starting at the cell where the iron concentration is only 25 g / l) aluminum, lead - or graphite cathodes.

The invention is further illustrated by the following examples.

EXAMPLE 1

Pickling in open containers of galvanized steel tubes, combined with electrochemical sulfuric acid regeneration.

Sulfuric acid solution is stained at 70 ° C for four open containers arranged in a flow direction one after the other. In the case of stationary equilibrium in the circular process, the contents of the individual containers are given the following concentrations:

Container No. HgSO 4 Fe ++ (g / l) 20 _ 1 262.5 30 2 195.0 50 3 127.5 70 4 60.0 90 25__

The concentration of ammonium sulfate in all four containers is 80 g / l.

The stain leaving the fourth vessel is continuously fed to the cathode space of the electrolytic cell line. The solution flows through the cathode compartment below which its iron content is excreted on the iron plates suspended in the liquid. As the solution flows through the cathode compartment, its pH increases to 1.8 and its iron content drops to 12 g / l. The ammonium sulfate content increases to 165 g / L. The anode compartment is separated from the cathode compartment by a stretched polypropylene tissue aperture. The cells are made of textile bakelite profiles, and as a seal, silicone rubber is used. As anodes, semi-hard lead plates containing 1% silver are used. The sputum catholyte is transported by means of a pump back to the beginning of the cell line and into the anode compartment, from which it flows through the anode compartment of the successive cells. Below this, the acid content of the solution rises to 294 g / L and the ammonium sulfate content is reduced to 80 g / L. Electrolysis is carried out at a current density of 18 A / dm and the temperature of the electrolysis vessels is maintained by changing the flow rate of the cooling water at about 85 ° C.

5 The final ethanol is added to sulfuric acid to replace the sulphate loss caused by electrolysis (sulphate removed with the stained blanks) until its acid concentration is 330 g / l. Thereafter, the regenerated acid is returned to the first pickling container and thus the circuit is closed. In the electrolysis or vessels, the evaporation loss is replaced by water 10 and the ammonium sulphate loss caused by electrolysis (ammonium sulphate removed with the stained blanks). The electrochemical regeneration achieves a power efficiency of 64% and the specific energy requirement for each 1 kg of extracted cathode iron is 6.1 kWh. The cathode plates are replaced daily. The cathode plates removed from the solution, which have taken the form of blocks as a result of 15 of the precipitation, are remelted.

EXAMPLE 2

Electrochemical processing of the used pickling solution resulting from the manufacture of iron rods combined with the preparation of iron-20 powder.

Discontinuous operation is carried out in independent open containers at 80 ° C. The used pickling solution contains per. 80 g of acid, 80 g of Fe ++ ions and 90 g of sodium sulfate. The design and operation of the electrolysis apparatus is identical to that described in Example 25 1. The final catholyte contains per 12 g of iron and 174 g of sodium sulfate. The final ethanol contains per 199 g of sulfuric acid and 90 g of sodium sulfate.

The electrolyzer consists of 16 series connected cell units. The stationary concentrations obtained in the 11th cell are as follows: 16 g of 30 sulfuric acid, 24 g of Fe ++ ions and 148 g per ml. liter. In the first 10 containers, as cathodes, iron sheets are treated which are treated in the manner described in Example 1. In the other containers, soft lead cathode plates are enclosed. These lead plates are taken out of the liquid for each layer one at a time and passed through a roller which bends the lead plate slightly.

35 In doing so, the pendent iron powder crumbles off and can be collected together. The cathodes are always removed from the solution under current. As several cathodes operate in each cell, the current is not interrupted by removal of a cathode plate. The current efficiency is at block iron 60% and at iron powder 66%. The energy requirement is for block iron of 7.0 kWh / kg and for iron powder of 6.4 kWh / kg.