In many surface technology processes, formed metal salts must be removed, e.g. from pickling or etching baths. While acids and metal salts or metal oxides are recovered or discharged in conventional bath maintenance processes - such as retardation, diffusion dialysis or roasting processes - electrolysis processes can be used to recover the metals and process acids.
In the conventional electrolysis of bath solutions, water is normally split into H+ ions and oxygen at the anode and the metal is separated at the cathode. In the conventional electrolysis of chloride solutions, however, chlorine gas is produced at the anode. Additional H2 is required here to recover hydrochloric acid.
H2 gas diffusion electrodes (H2-GDE) are currently being optimized for use in fuel cells (FC). The efficiency for electricity generation from H2 in the FC is in the range of 50 % in normal operation in relation to the lower calorific value of hydrogen. The use of H2 in new electrolysis processes with H2-GDE for the recovery of metals and acids can substitute significantly more electricity than would alternatively be generated in a FC. Due to the overvoltage of O2 in conventional electrolysis, more than 100 % of the energy content of H2 can often be substituted in electricity compared to conventional electrolysis.
The recovery of metals - e.g. Zn, Fe, Co, Ni, Sn, Pb, Cu - by electrolysis with H2-GDE from process baths thus leads to significantly reduced costs for electrical energy compared to conventional electrolysis and also offers significant ecological and economic advantages in bath maintenance compared to other recovery processes. Due to their outstanding industrial importance, iron pickling baths are discussed below.
Pickling baths for iron/steel
Steel is usually pickled in hydrochloric or sulphuric acid baths. Pickling here means the removal of corrosion layers on surfaces or the removal of cover layers from previous processes, e.g. rolling or annealing scale. When pickling with mineral acids, iron oxides dissolve, but some of the base metal is also attacked with the formation of hydrogen. Iron(III) salts formed also react further with iron to form iron(II) ions. Most of the iron(II) salts are therefore dissolved in the pickling acid. Hydrochloric acid dissolves all iron oxides completely, sulphuric acid dissolves FeO easily, Fe304 poorly and Fe203 hardly at all [1]. Iron pickling with hydrochloric acid is therefore preferred. For Fe3+ salts, 2 mol HCl/mol dissolved Fe are also required here, as for Fe2+.
Fe2O3 + Fe + 6 HCl → 3 Fe2+ + 6 Cl- + 3 H2O<1>
The pickling effect depends on the condition of the pickling bath. In particular, the drop in the acid level and the rise in the iron level interfere with pickling, with the latter having the greatest influence on pickling times and the pickling effect. The drop in the acid level is counteracted by re-sharpening with fresh acid. The result is processed baths with a relatively high amount of iron and a relatively high acid content [2].
Concentrations above 1 mol/l iron ions in spent pickling solutions prevent the efficient use of modern acid recovery processes such as nanofiltration and/or diffusion dialysis for iron pickling due to the high osmotic pressure. In practice, iron pickling solutions in smaller plants are therefore still discarded after reaching the maximum iron concentration in the range of 50 - 100 g Fe/l, or treated by retardation with acid recovery rates of up to 90 % for the free acid. However, the free acid only makes up a small proportion of the amount of acid originally used. The amount of wastewater is also increased by over 100 % when retardation is used compared to discarding the solution [3].
In larger hydrochloric acid pickling plants, the spent pickling solution is treated using a roasting process [4]. In this process, the liquid and the free HCl evaporate first, then the metal chloride is converted into oxide with the formation of further HCl.
3 FeCl2 + 3 H2O+ ¾ O2 → 1.5 Fe2O3 + 6 HCl <2>
The HCl in the waste gas is washed out and fed back into the pickling process. The resulting products are iron oxide and hydrochloric acid. In contrast to retardation, almost 100% recovery of all the hydrochloric acid used is achieved, albeit with high energy input and equipment costs. Assuming a high Fe concentration of 1.5 kmol/m3 solution, approx. 12m3 of solution must be evaporated per t of Fe discharged, resulting in an energy requirement for evaporation in the region of 10 MWh/t Fe.
Regeneration of iron pickles with electrolysis
The regeneration of hydrochloric acid baths using conventional electrolysis with chlorine gas formation at the anode
Fe2+/Fe// 2Cl-/Cl2idealcell voltage Δ_Eo = 1.77 V <3>
requires a real cell voltage in the range of 2 V (neglecting the overvoltage of chlorine) with a resulting energy requirement in the range of 2 MWhel per t of deposited iron. In order to recover the hydrochloric acid, an additional reactor for HCl formation is also required, in which 1.2 MWh of H2 must be used to form 1.3 tons of HCl. Compared to roasting processes, this results in a reduction in energy requirements of approx. 10 MWhth to 2 MWhel + 1.2 MWh H2 = 3.2 MWh/t Fe. The acid is also completely recovered and a higher-value product is obtained in the form of iron as iron oxide. The iron formed is carbon-free and ideally suited for further processing into stainless steel.
Significant additional savings are achieved if hydrogen is used directly in the new electrolysis with H2-GDE:
Fe /Fe2+//2H+/H2
ideal cell voltage Δ_Eo = 0.45 V<4>
The losses in the electrolysis are estimated at approx. + 0.3 V, as H2 only has a minimal overvoltage on platinum-doped H2-GDE. The voltage required for one cell is approx. 0.75 V, and 0.8 V if the depletion of Fe2+ ions during electrolysis is taken into account. This results in an energy requirement of 1.2 MWh H2 and < 0.6 MWhel per ton of Fe recovered. The entire HCl consumed in the pickling process (2 mol/mol Fe, approx. 1.3 t HCl/ t Fe) is now recovered directly in the electrolysis process, without additional equipment.
From an energy point of view, the use of electrolysis with H2-GDE is therefore highly attractive for the treatment of iron pickles with the recovery of iron. Compared to roasting processes, the energy requirement is reduced from approx. 10 MWhth to 0.6 MWhel + 1.2 MWh H2 = 1.8 MWh per t of Fe extracted. Compared to conventional electrolysis, the energy requirement is reduced from approx. 2 MWhel + 1.2 MWh H2 = 3.2 MWh per t to 0.6 MWhel + 1.2 MWh H2, a total of 1.8 MWh per t of Fe discharged. The use of 1.2 MWh H2 directly in the electrolysis thus leads to a saving of 1.4 MWhel compared to conventional electrolysis.
Fig. 1: Flow diagram for metal and acid recovery from iron pickling
Overall process for regenerating iron pickles with H2-GDE electrolysis
In addition to acid and iron salts, iron pickles contain other impurities such as solids (e.g. silicates), oils and fats and other dissolved metal salts. Figure 1 shows a process flow diagram for the continuous recovery of iron and acid from a pickling solution by membrane electrolysis with H2-GDE. The iron pickling process can be operated in a stationary state and an optimum bath composition can be set for the pickling process. For this purpose, a material flow is continuously fed from the pickling solution via microfiltration to the cathode chamber of a membrane electrolysis with H2-GDE. Oils, fats and solids are removed in the microfiltration before iron is deposited at the cathode in the electrolysis. The corresponding quantity of chloride ions diffuses through the anion exchange membrane into the anode chamber, where the acid is formed from these and H2.
The solution emerging from the cathode chamber still contains hydrochloric acid with the same acid concentration as in the pickling solution, a residual content of Fe ions and other metals (Zn, Al etc.) that are difficult or impossible to separate electrolytically.
A partial flow of the solution emerging from the cathode chamber is fed to a diffusion dialysis with an anion exchange membrane to remove the foreign metals while simultaneously recovering the acid. Here, the acid is depleted by counter-current water and an effluent is obtained in which approx. 10 % of the original acid quantity and the dissolved foreign metal salts are still present. These can be precipitated as metal hydroxides by adding alkalis or burnt lime, for example.
The diffusate from the diffusion dialysis is returned to the anode chamber together with the remaining solution flowing out of the cathode chamber and finally the solution regenerated there is returned to the pickling bath.
The iron deposited on the cathode is washed with water and the acidified water is fed to the diffusate channel of the diffusion dialysis. The waste water 2 contains only a minimal amount of chloride ions, so almost all of the chloride is returned to the pickling solution as hydrochloric acid.
Table 1 summarizes a rough economic analysis for the new process. This assumes an iron quantity to be discharged of approx. 1 t/h = 5 mol/s with a bath composition of 60 g/L Fe and 15 % HCl. The loss of iron and acid in the wastewater is neglected for the sake of simplicity. Approx. 18m3/h of bath solution is fed to the electrolysis, in which the iron content of the solution is reduced to 4 g/L. The cumulative current strength across all H2-GDE is approx. 1 MA, which means that approx. 200m2 of H2-GDE surface area is required at 500 mA/cm2. The costs of the electrolysis plant are assumed to be approx. 4000 Euro/m2 H2-GDE. The costs can be halved in the future if H2-GDE membrane units are mass-produced for fuel cells.
|
Investment |
2.5 million euros |
|
Annual operating costs |
2 million euros/a |
|
Sale Fe |
3 million Euro /a |
|
Savings on hydrochloric acid |
1 million euros /a |
Approx. 10 % = 2m3/h of the solution is fed into the diffusion dialysis to remove the foreign metals. Spiraltec GmbH's spiral coils have a capacity of 20 l/h for hydrochloric acid applications. This means that 100 coils are required. The cost of diffusion dialysis is assumed to be 4,000 euros per coil.
The investment for the entire plant is estimated at 2.5 million euros, of which approx. 1.2 million euros are accounted for by the main components membrane electrolysis with H2-GDE and diffusion dialysis.
If an operating time of 6000 h/a = 6,000 t Fe/a is assumed, this results in an annual demand for hydrogen of 7,200 MWh and for electricity of 3,600 MWh. The energy costs result at prices of 100 Euro/MWh each at approx. 1 million Euro/a. With this plant size, H2 can be provided by steam reforming of biogas with 1.2 MW output without resulting fossilCO2 emissions [5]. In the future, very cost-effective H2 generation can also be achieved by pyrolysis of natural gas in order to achieve costs < 50 Euro/MWh H2 [6]. The total annual variable operating costs are conservatively estimated at 2 million euros, operating costs of less than 1 million euros/a are probably achievable with favorable energy costs.
At prices of 500 euros/t Fe carbon-free (carbon steel approx. 800 euros/t [7]), this results in a turnover of approx. 3 million euros/a. In addition, 8,000 t/a of HCl are saved for pickling, with a value of approx. 1 million euros.
Based on this simple and conservative estimate, a very positive assessment can be made not only for the energy benefits but also for the economic efficiency of the overall process with an operating result of approx. 2 million euros/a with investments of 2.5 million euros/a.
Outlook and further applications
The predicted advantages for the treatment of iron pickles with H2-GDE and diffusion dialysis should be verified in practice in the next step. In addition to iron, the new process can then also be used to recover other electrolytically depositable metals from acidic solutions with simultaneous acid recovery, e.g. Zn, Fe, Co, Ni, Sn, Pb, Cu. Due to the higher prices for all these metals, even higher profitability is expected here. Other acids such as sulphuric acid or nitric acid or mixed acids with metal salts, e.g. stainless steel pickles, can also be processed using the new method. In a further step, metals could also be selectively deposited from mixtures of different metal salts using several electrolyses connected in series with increasing voltage.
In addition, the new electrolysis process can also be used in future for efficient metal extraction from ores withoutCO2 emissions, once the advantages in the processing of iron pickles have been verified. Conventional oxygen steel production requires approx. 5 MWh of fuel and a further 0.4 MWh of electricity per tonne [8]. If iron ores are alternatively dissolved in acid and the new process is used for metal extraction and acid recovery, the energy requirement for steel production (C-free!) is reduced to 1.2 MWh H2 and 0.6 MWh electricity, even with a large-scale H2 supply through pyrolysis of natural gas withoutCO2 emissions. This results in a highly efficient cold blast furnace without gaseous or noise emissions for the production of high-quality steels.
Literature
[1] A. Hake, Wasser und Abwasser, Vienna 1967, p. 109 ff.
[2] Th. Borgolte, Galvanotechnik 57 (1966) No. 8, p. 531 ff.
[3] K. Klein, Galvanotechnik 85 (1994) No. 1, p. 210 ff.
[4] Patent EP 1 253 112 B1
[5] www.dbfz.de/fileadmin/user_upload/Referenzen/DBFZ_Reports/DBFZ_Report_46.pdf
[6] P. Lott et al, ChemSusChem 2023, 16, e202201720.
[7] www.stahlpreise.eu
[8] www.bmwk.de/Redaktion/DE/Downloads/E/energiewende-in-der-industrie-ap2a-branchensteckbrief-stahl.pdf
