The pellets are next advanced to the cooling zone Where gases are introduced to the windboxes 23 and 24 and collected in a common hood 17. Windbox 23 is supplied with a portion of the gases from the scrubber and cooler 39 through line 42. These gases contain a smallv quantity of water vapor, a variable small percentage of oxygen, up to about 7% carbon monoxide, about 10'- 12% carbon dioxide, and the balance nitrogen,<a href="http://www.sellercrusher.com/project/zenith-mining-machines.html">zenith mining machines</a> which composition is essentially non-oxidizing at a flow rate of about 200-300 s.c.f.m. per square foot of grate area and a typical gas temperature of about to F. The windbox 24 receives another portion of the gases from scrubber and cooler 39 through line 43, mixed with controlled amounts of atmospheric air from line 47 and blower 46. The gases passed through the bed at a flow rate of about 400-600 s.c.f.m. per square foot of grate area are essentially non-oxidizing to the pellets which have been substantially cooled in the first stage cooling to a level where exposure to air will no longer oxidize the iron. Total time in the cooling zone is about 6 minutes at the above flow rates and gas composition. A portion of the gases collected in the hood 17 are recirculated to the burner 32 in the downdraft preheating and firing zones through line 34, and another portion is recirculated to the downdraft drying zone through duct 28.
The pellets are finally discharged from the traveling grate to a suitable container or conveyor (not shown) for later use in a blast furnace or other conversion furnna ce as desired. The final pellet product has been reduced by the process to the extent that 5095% of the original combined oxygen is removed from the iron oxides.Further understanding of the invention may be gained by a fuller explanation and recapitulation of the heating and cooling cycles nccuring in the pellet bed.<a href="http://www.sellercrusher.com/project/feldspar-grinding-machine.html">feldspar grinding machine</a> In the preheating zone the gases passing through the bed are the hottest and the highest in oxygen. The temperature of the uppermost layer of pellets rises sharply to their maximum temperature by the exothermic reaction of the coating fuel and oxygen while the pellets below that level rise less sharply so that they reach their highest temperature at a later time. Most of the coating fuel on the upper level pellets is burned off in the preheating zone and the heat generated thereby is carried to the lower level pellets by the heated and now oxygen-poor gases. When the pellets enter the downdraft firing zone the oxygen level of the gases is reduced and the uppermost layer of pellets is maintained at reducing temperature.
The heat carried off from these pellets is carried to the lower levels of pellets so that the pellets just below the topmost layer reach their maximum temperatures in the first portion of the downdraft firing. In the succeeding portion of the downdraft firing zone the gases entering the bed are further reduced in oxygen content, thereby minimizing the reoxidation of the topmost pellets and the subjacent pellets whose coating fuel has been substantially completely burned by this time. The heat carried from these pellets further heats the lower levels of the bed as the pellets advance into the updraft firing zone. The oxygen level of the gases is again increased for updraft firing in order to burn the coating fuel on the lowermost pellets raising them to their maximum temperature, and the combustion gases from the oxidation of the coating fuel pass upwardly to further heat the superjacent pellets and further heating the pellets in the upper levels whose coating fuel has been consumed.