Storage and transportation for direct reduced iron

1. The nature and passivation of DRI

DRI is the normal name for direct reduction process products. Iron oxide in the reactor at a lower temperature solid state get reduction, due to the oxygen in the exhaust and reducing swelling, reduced product has a porous, porosity up to 50% to 70%, the internal specific surface area up to 1000 ~ 3000cm2/g, at 1,000 magnification of the electron microscope picture, you can clearly see "cavernous" structure. Therefore, the product discharged from the reactor narrowly as "sponge" is called "sponge iron pellets" or "metal pellets"

Direct reduction process sponge iron exist the disadvantage of re-crystallization incomplete, there is a serious crystalline defect, such as the absence of distortions, and grain boundary disorder and so on. The crystal defects make the sponge iron actively and easily oxidation. The porosity and large specific surface area create condition for diffusion and oxidation reactions for oxidizing gas .This sponge iron at a certain temperature and humidity conditions of the oxidation reaction will occur again. By electrochemical corrosion theory, the oxidation reaction is the non-polarization caused by the negative region, the reactions as follows

Mechanism analysis showed that: in the dry air temperature, the iron oxidation reaction in the atmosphere is slow. Direct reduction iron oxide quickly and then combustion, it is because stockpile with wet water, sponge iron re-oxidation heat generated by the reaction cannot be quickly dissipated. It has proved that each absorb oxygen from about 0.1% to 16.751 emit heat, under adiabatic conditions, the temperature rise 35 ℃. Once cause heat accumulation, temperature, and then the chain oxidation reaction will lead to a direct reduction iron stockpile of spontaneous combustion, the "fire" phenomenon. The main impact is the sponge iron oxides and hydroxides of iron oxide

A large number of experimental studies have confirmed that direct reduction and oxidation is determined by raw materials, process methods and operating conditions.

To avoid direct DRI re-oxidation even spontaneous combustion during storage and transportation process, it should be passivated, eliminate its activity by physical or chemical means. There are several ways.

① DRI compacts get into a block of inert gas at high temperature and pressure, reduce pores, eliminate activity. There are 89mm × 38mm × 13mm and 92mm × 38mm × 25mm direct reduction on abroad, briquetted iron becomes dense (density 5 ~ 6g/cm3). Abroad have done the people buried in the red-hot ingots direct reduced iron briquette heap test, which did not produce combustion phenomenon, we have used the spray dedusting method in direct reduction iron loading and unloading, and it did not show oxidative exothermic reaction, the metalized rates were not significantly reduced.

②Directly cooling passivation      Use the cooling water directly cooling the hot DRI pellets. So on the surface of the pellets will form a thin layer film of Fe3O4 to decrease the activity of the DRI pellets. This can protect the DRI pellets against from re-oxidation at storage and transportation. 

③ Aging passivation   The obtained sponge iron pellet production at higher temperatures, stored in a well-ventilated space, thickness not more than 1.5cm, the generated heat can be dissipated rapidly aging. Storage for two days, sponge iron pellets metallization rate will decline less than 1%. Sponge iron pellets through natural oxidation process re-passivation then storage and transport, which can effectively prevent further oxidation.

In addition covered with plastic film or coating tar can also preventing re-oxidation effectively, but required high cost, about 20% of the production.

Direct reduction iron density and natural angle of repose in Table 1.

Tab.1 Direct reduction iron density and nature repose angle

2. DRI loading and unloading, storage and transport

(1) Loading and un loading

All loading and unloading solid materials equipment can be directly reduced iron, such as large grab, loaders, cranes and other electromagnetic. Loading and unloading equipment is used in large-scale iron ore transport, such as electromagnetic cranes and other, the  damage of direct reduced iron is slight, using the scrap handling equipment, such as cranes and ，but costs is larger.

DRI's inbound and outbound can use continuous handling and transportation equipment; and can also be used an automatic conveyor, directly put the DRI into furnaces, or you can use the vibrating feeder, put the DRI into the scrap tank, the same as processing scrap

It has been found abroad that DRI load and unloading must raised dust. Dust was black, with a clear visibility, and can drift considerable distances, causing serious pollution to the environment. The dust contains a lot of metal powder; when it fells on objects and equipment surfaces will show its chemical activity, erosion paint, work and metal structures. This is a difficult problem to solve; the current approach is to strengthen the handling properties of water work.

The designed DRI loading and unloading and transportation programs should avoid DRI contact with water, reduce handling gap, reducing transfer times, and avoid direct reduced iron pressed by grinding.  These works will undoubtedly reduce the dust and breakage of direct reduction iron.

 (2) Storage

The foreign countries have done a lot of experiments and accumulated a wealth of practical experience for DRI library storage and storage sites. Direct reduced iron with passivation process once take protective measures, it does not overheat and the metallization ratio is not significantly reduced.

The new product sponge iron pellets without passivity must be stored in a covered ventilated location aging for treatment. After DRI aging treatment DRI need to keep dry, store in a covered warehouse or store in open area covered with canvas. Foreign proved that DRI through treatment, stored on dry, flat, well-drained open area, although over some time, but the surface layer forming a crust rust on the surface of the metal and has a certain loss rate, but the crust below the lower rarely. Figure 1 gives the relationship about sponge iron pellets stored in unprotected open space for direct reduced iron briquettes the metallization loss rate and stockpiling time.

Curve shows that the sponge iron pellets surface metallization rate is serious loss, and direct reduced iron briquettes surface the metal the rate loss is higher than the inside, but not very significant. Overall, the rate of metal sponge iron pellets loss is greater than the direct reduction of iron briquettes. Another test confirmed that 3m high pile tower after 200 days the average deposit metal rate loss the direct reduction iron was 2.1%, 7% sponge iron pellets, after heightening the pile some average metal loss rate decline

 (3) Transportation

On abroad, the all transport means for direct reduced iron used maritime, land. Practice has proved that as long as take certain protective measures, ships, trains, cars can be safely transported DRI. When shipping DRI for safe transportation, we should do the following preparations, the cabin must be cleaned to remove all dirt, greasy and acidic debris with organic matter, hatch covers watertight cabin drying no condensate; Do not loading and unloading DRI in rain, snow or fog weather; direct reduced iron silos of the material layer should be set thermocouple, to monitor the temperature of the material layer at any time. Thermocouples inserted at 15 ~ 30cm cutting layer, once the material layer temperature exceeds 100 ℃,it will stop for inspection. We should do the following preparations by train or car transportation DRI: use enclosed trolley to prevent air circulation between materials; bottom and sides of the train must be waterproof, do not let water to seep into the interior; use sheds car top canvas cover, do not be leaking.

Foreigner have done DRI test with 7.5mm × 7.5mm × 3.5mm container shipping, the water or seawater added to the DRI, no heat generation and generation of hydrogen phenomenon. Direct reduction through compact passivation has conducted several ocean shipping, transportation proved to be safe, the metal loss rate is very small

Mexico had the 270,000 tons sponge iron pellets used Sheds train transportation 1126km, after 4-8 days, sponge iron pellets metallization rate from 85.62% down to 84.6%. Stability of these products are related to the reducing high operating temperature, carbon content of 1.5% to 2.0%, and other factors.

Practice has proved that the direct reduction iron storage, loading and unloading and transportation does not exist any problem of insecurity, but it should be noted that DRI has the activity; we cannot ignored storage, handling and transportation security issues.