“FROZEN FURNACE” OF HYDROGEN REDUCTION OF IRON ORE BLUE DUST CONCENTRATE, ARCI, HYDERABAD, INDIA.
Figure 1 Hyderabad of Telengana state, India is among picturesque rocks of Deccan plateau. It is Southern India. Hot and dry weather. There is Advanced Research Center International for powder metallurgy and advanced material. The study had been done within frame of Scientific Colalboration Ukraine – India, 1998
WHAT HAD BEEN DONE?
The reduced under hydrogen iron powder from blue dust has a lot of advantages against two stages carbothermic technology process.
Specific energy consumption is acceptable. No problem with exhaust gases after liquefied gas burners as CVSF process with low fuel energy efficiency.
The powder is very ductile, with sufficiently well compressibility at load of 4.2 t/cm3 and extraordinary green strength for 6.5 g/cm3 compressed density (no lubricant added) 65 N/mm2,which in time higher than standard market sponge iron powder.
It has low apparent density 1.29 g/cm3, but steel acceptable flow ability 74 seconds of 50 g standard sample.
Oxygen content of 0.4% is slightly higher than standard market sponge iron powder, but it is possible to improve this characteristics.
Carbon content of just 0.007% suggests that by hydrogen reduction of blue dust ore concentrate is possible to produce a unique carbon free iron powder.
To increase apparent density and make reduction process more flexible it is suggested to divide reduction process in two stages.
The first stage is to end with oxygen content 2 – 3%, because especially at this level the reduction decelerates sharply and apparent density is stabilized.
Intermediate operations for additional purification of the powder by magnetic separation of non-metal irreducible inclusion to be removed and operation to increase apparent density could be included.
The second stage reduction under hydrogen would start again with powder layer, not sintered crust and reduction process may be accelerate again to decrease specific energy consumption even more.
The reduced powder is very sticky and easily welded to the boat surface. So special device to cover the boat surface with carbon protective layer was invented, and Indian patent claimed and issued No 211922
Fig.2. Results of "Frozen Furnace" experiment
Results of “Frozen Frunace” experiment is shown in Fig 2. First period from 0 to 100 minutes was named “induction period”. During this period the reduced and sintered upper part of layer were not observed. Reduction process proceeds in whole volume of powder bed. It seems, powder bed is well penetrated by gas during this period and there is no limitation by diffusion during the period. That is why there is no difference between upper and bottom oxygen content, but average oxygen content decreases sufficiently fast. Then, by the end of period, between 80 and 100 minutes a fresh reduced top crust is created and diffusion of fresh hydrogen through powder bed becomes a limitation of process. It begins a second period.
The second period from 100 to 180 minutes was named “transferring period”. During this period reduction of bottom part of layer sharply slows down, difference of oxygen content between upper and bottom appears and the difference between them grows quickly. At the same time the oxygen content of upper layer decreases and by the end of period stabilizes on level around 2.0% (previously calculated level). That means the reduction proceeds preferentially in upper sintered part of layer. Due to decreasing of upper layer oxygen content and the growth of upper sintered layer the average oxygen content decrease too. By the end of the period the average oxygen content and the difference between top and bottom are stabilized.
This is the beginning of a third period. It is the “frontal period”. The oxygen content of upper and bottom parts of layer are constant from 180 to 280 minutes, the difference between them is constant too. However, average oxygen content is gradually decreasing because the thickness of upper sintered crust is gradually growing for the period. That means the reaction takes place only in new created boundary layer, which is gradually moving towards bottom of boat. It may be explained by achieving equilibrium between fresh gas stream penetrating to reaction boundary and waste gas (H2O) stream going through upper sintered layer of the powder.
Step by step the thickness of upper sintered layer increases and thickness of bottom decreases. By 280 minutes bottom part has become so thin that begins the latest fourth period, namely the “final period”. After 280 minutes it may consider that difference of top and bottom disappears. It can observe sharply growth of upper crust thickness and decreasing the difference among top, bottom and average oxygen content. Process has being completed, because the oxygen content becomes uniform in whole powder layer volume. It is defined by the equilibrium between controlled atmosphere and FeO of powder.
Apparent density of the reduced iron powder is very significant characteristics. For our purpose apparent density of the upper sintered crust is the most significant, because it determine changing of apparent density for reduced powder.
THE TECHNOLOGY CAN BE REALIZED WITH MODERNIZATION AND MODIFICATION AGAINST AN ORDER.