Reaction SMILES: [C:1]([OH:6])(=[O:5])[C:2]([OH:4])=[O:3].[Fe:7]>C([O-])(=O)C([O-])=O>[OH2:3].[OH2:3].[C:1]([O-:6])(=[O:5])[C:2]([O-:4])=[O:3].[Fe+2:7]
Reactant: O=C(O)C(=O)O
Reactant: [Fe]
Agent: O=C([O-])C(=O)[O-]
Product: O
Product: [Fe+2]

Procedure:
Thus, in a preferred embodiment of the invention, a method is provided for reprocessing spent pickling waste liquor, comprising adding oxalic acid and so sequestering about 75-90% of iron in insoluble oxalate, washing said solid iron oxalate by water till the pH of 4.5-7 in the wash water, drying said washed iron oxalate at 90-105° C. thereby obtaining a powder of iron oxalate dihydrate, and calcining the dried iron oxalate dehydrate. In the most preferred embodiment, the process further comprises washing said solid iron oxalate by water till the pH of the wash water is about 4.5-7 in, drying said washed iron oxalate at 90-105° C., preferably obtaining iron oxalate dehydrate, and calcining the dried iron oxalate at 200-300° C. in the atmosphere of dry air, thereby obtaining a red powder of nano-sized α-hematite. The aqueous galvanic waste sludge usually consists of 10-15 g/l of hydrochloric or sulfuric acid, 3-5 g/l of inhibitor additive, and 115-150 g/l of Fe2+ as ferrous chloride or ferrous sulfate. The sludge is preferably filtered and filled into acid resistance reactor with stirrer at ambient temperature, and it is mixed with quantity of dry oxalic acid dihydrate (99.6%) stoichiometric to 75-90% of Fe2+. After adding oxalic acid, and stirred for a time sufficient for producing iron oxalate, usually about 4 hours, iron (II) oxalate dihydrate precipitates, whereby regenerated hydrochloric or sulfuric acid is formed. The regenerated quantity of hydrochloric or sulfuric acid is about 75-80% of spent pickling acid, containing residue of 30-50 g/l of Fe2+ and 3-5 g/l of inhibitor additive. The regenerated hydrochloric or sulfuric acid is separated from slurry by filtration together with dissolved inhibitor additive and it is reused in the pickling process. The precipitated iron (II) oxalate dihydrate is separated from slurry by filtratioin, producing a filtration cake, to be further processed. The cake of iron (II) oxalate is is preferably washed by water till pH 4.5-7, and then it is preferably dried at a temperature of from +90 to +105° C. The dried cake of iron (II) oxalate dihydrate is calcined in an oven at 200-300° C. during 3 h in the dry air atmosphere, producing red nano-sized iron oxide (αFe2O3-hematite). The iron oxide (αFe2O3-hematite) need not be milled, it has a bulk density of 0.35-0.4 kg/l. The particle size is 35-50 nm at narrow distribution, 90% of the particles mass being in the said range. The purity of the product is usually 99.2-99.7%. Usually, iron oxide (αFe2O3-hematite) light powder consists of spherical particles. The said dried cake of iron (II) oxalate may be calcined in an oven at +290-350° C. over about 3 hours in the atmosphere of its own conversion gases, or alternatively at +450-500° C. in the nitrogen atmosphere, providing a black powder of super paramagnetic nano-sized iron oxide (Fe3O4-magnetite). The purity of the product is usually 99.5-99.8%. The iron oxide needs no milling, its bulk density being 0.45-0.5 kg/l, and its particle size being usually about 30-60 nm, at narrow distribution, about 90% particle mass being in said range. The powder usually consists of spherical particles. In a preferred embodiment of the invention, CO2 gas obtained in the iron (II) oxalate thermal decomposition is absorbed in NaOH. In other preferred embodiment of the invention, CO gas obtained in the iron (II) oxalate thermal decomposition is burned and the formed CO2 is absorbed in NaOH. The Na2CO3 obtained as a result of gases absorption is a by-product for sale.