those produced as by-products in industrial processes such as the acid vat treatment (pickling) of steel):

Oxidation of Fe metal in the Laux process where nitrobenzene is treated with iron metal using FeCl2 as a catalyst to produce aniline:C6H5NO2 + 3 Fe + 2 H2O   →   C6H5NH2 + Fe3O4Oxidation of FeII compounds, e.g. the precipitation of iron(II) salts as hydroxides followed by oxidation by aeration where careful control of the pH determines the oxide produced.Reduction of Fe2O3 with hydrogen:
3Fe2O3 + H2 → 2Fe3O4 +H2OReduction of Fe2O3 with CO:
3Fe2O3 + CO → 2Fe3O4 + CO2Production of nano-particles can be performed chemically by taking for example mixtures of FeII and FeIII salts and mixing them with alkali to precipitate colloidal Fe3O4.
This process is described by the Schikorr reaction:

  
    
      
        
          
            
              
                3
                
                Fe
                
                  
                    (
                    OH
                    )
                  
                  
                    2
                  
                  
                    
                  
                
              
              
                ferrous
                 
                hydroxide
              
            
          
          ⟶
          
            
              
                
                  Fe
                  
                    3
                  
                  
                    
                  
                
                
                  O
                  
                    4
                  
                  
                    
                  
                
              
              magnetite
            
          
          +
          
            
              
                H
                
                  2
                
                
                  
                
              
              hydrogen
            
          
          +
          
            
              
                2
                
                
                  H
                  
                    2
                  
                  
                    
                  
                
                O
              
              water
            
          
        
      
    
    {\displaystyle {\ce {{\underset {ferrous\ hydroxide}{3Fe(OH)2}}->{\underset {magnetite}{Fe3O4}}+{\underset {hydrogen}{H2}}+{\underset {water}{2H2O}}}}}
  This works because crystalline magnetite (Fe3O4) is thermodynamically more stable than amorphous ferrous hydroxide (Fe(OH)2 ).The Massart method of preparation of magnetite as a ferrofluid, is convenient in the laboratory: mix iron(II) chloride and iron(III) chloride in the presence of sodium hydroxide.A more efficient method of preparing magnetite without troublesome residues of sodium, is to use ammonia to promote chemical co-precipitation from the iron chlorides: first mix solutions of 0.1 M FeCl3·6H2O and FeCl2·4H2O with vigorous stirring at about 2000 rpm.