Direct reduction of iron is the reduction of iron oxide in the solid state at a temperature below the melting point. Due to the oxygen discharge and reduction expansion, the reduction product is porous, the porosity can reach 50% ~ 70%, its internal specific surface area is more large, due to porosity and large specific surface area, for the diffusion of oxidizing gas and oxidation reaction to create the conditions, so the sponge iron is active, at a certain temperature conditions, easy to re-oxidation reaction.
Experiments show that for every 0.1% oxygen absorbed by sponge iron, under adiabatic conditions, the temperature rises about 35 ⊥. The heat accumulation will cause the temperature to rise and the oxidation reaction will then lead to spontaneous combustion of the direct reduced iron stockpile. In order to avoid reoxidation or even spontaneous combustion of direct reduced iron during storage and transport, passivation should be carried out, i.e. its activity should be eliminated by physical or chemical means.
Passivation treatments are available as follows.
- Direct reduced iron is pressed into a block under high temperature and inert gas protection to reduce porosity and eliminate activity, and the straight reduced iron becomes dense after briquetting. The strength of the direct reduced iron briquette will affect the amount of friction and impact resulting in broken pieces and powder.
- Direct cooling passivation, the high temperature sponge iron pellets discharged from the reduction reactor, sprayed with water for direct cooling, forming a very thin layer of Fe3O4 film on the surface of the sponge iron pellets, reducing the activity of the sponge iron pellets, which can effectively avoid reoxidation during storage and transportation.
- Aging passivation, the hot sponge iron pellets, stored in a well-ventilated site, the thickness of the pile should not be greater than 1.5cm, so that the heat generated by aging can be quickly dissipated.
- The packaging bag with moisture-proof plastic film can also prevent re-oxidation of the DIR products.
