Drying and gas or vapor contact with solids – Process – Diverse types of drying operations
Reexamination Certificate
2001-05-08
2002-07-23
Esquivel, Denise L. (Department: 3744)
Drying and gas or vapor contact with solids
Process
Diverse types of drying operations
C034S508000, C034S509000, C034S510000, C034S210000, C034S230000
Reexamination Certificate
active
06421931
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to drying processes, and more particularly to a method and apparatus for drying iron ore pellets.
BACKGROUND OF THE INVENTION
Several processes have been in use over the years for drying green, i.e., moist, iron ore pellets, e.g., hematite, magnetite or limonite. The objective of these processes is to remove residual moisture so as to produce a strong fired pellet having maximum abrasion and breakage resistance as adjudged by crushing tests, optimum porosity and, where stored in cooler climates, good resistance to repeated freezing and thawing. In treating certain ores the process should also provide optimal oxygenation, since poor strength may otherwise result in the case of magnetite pellets where oxidation to Fe
2
O
3
is not complete, leaving magnetite cores in the center of the pellets.
Prior methods employed in drying iron ore pellets will now be described briefly by way of example in connection with the drying of magnetite pellets obtained from taconite. It should be understood, however, that although the present invention is described in connection with a particular ore, it is not limited to specific apparatus or processes described.
For the last 45 years, the beneficiation of magnetite-containing rock has consisted of crushing, grinding and milling the ore. The specific operation consists of separating the desired material from the gangue (waste) material through hydraulic separation, magnetic separation, and by chemically treating the ore to further enhance the separation of the ore from the waste rock.
The material separated from the waste material is called concentrate. The total iron may range from 65% to 69% or other economically practical value. The concentrate is generally described as a powder with the general size that can pass through a screen of a selected size. The screen usually used is an U.S. Standard Tyler Screen of 325 and 500 mesh to the inch. The 500-mesh screen has openings about 27 microns in diameter.
Some of the general size descriptions might be 85% minus 325 mesh and 75% minus 500 mesh as an example. The percentage values correspond to the amount of grinding necessary to liberate the desired product from the waste product. The grinding, milling and treatment of the ore generally occur in a section of the plant called the concentrator, hence the name concentrate.
The concentrate is generally piped in an aqueous slurry of 60% solids to a vacuum filter. The vacuum filter removes most of the water from the slurry. The resulting product is called a filter cake with generally less than 10% water. The amount of water is controlled by the efficiency of the filtering operation and also by the size of the particles in the concentrate. The concentrate (filter cake) is generally conveyed to storage bins before being fed into a disk or drum-balling device.
The concentrates have additives to improve the balling, firing or chemical composition of the product once it has been fired. Some of the common additives are bentonite clay, limestone in the form of calcium hydroxide if fluxed pellets are produced, and sometimes an organic binder.
The balling of concentrate is accomplished in a process in which the material is rolled in stages that increase the size of the pellet by applying a layer of concentrate upon a smaller pellet until the pellet reaches the desired size. The product from a balling drum is screened to selectively size the product. The undersized material is circulated back into the balling drum. The circulated material is called seed pellets. The balling action applies the concentrate to minimize interstitial spaces; hence, smaller particles are forced between larger particles. The mixture of particle sizes makes a pellet of maximum density. The additives also fill the interstitial spaces and often provide a pathway for the gradual removal of water from the inside of the pellet. Pathways are also provided for oxygen to enter the inside of the pellet during the firing of the pellet. Knowledge of the removal of water from the inside of pellets is necessary to appreciate the contributions that the present invention provides towards the firing of magnetite pellets. An adequate preliminary description of the equipment and the mineral beneficiation process has been provided. It is also necessary to describe the physical and chemical changes in each section of a pelletizing machine.
The prior drying process and some of the limitations of that system which negatively impact on the next stage of the pelletizing process (the firing of the pellets) will now be described. It should be noted, however, that even a detailed explanation of the physical changes of the product is an oversimplification of a complex process.
The finished pellets are screened and placed on conveyor pallets each having grate bars at its bottom that holds the pellets as they travel through the furnace. The pellets are placed gently on the pallet grate bars to form a level bed of pellets at a depth that has been established through practical experience. The depth is usually about 15 inches or more in thickness. Quite frequently, a layer of recently fired pellets is first placed upon the grate bars to form a layer of fired pellets about 3 inches thick. The fired pellet layer is called a hearth layer. Each pallet is part of an endless track conveyor about 300 feet long and often 8 to 12 feet wide. One common conveyor is called a traveling grate machine. The conveyor is part of and contained for the most part within the drying, firing magnetite conversion and cooling zones of a furnace.
There are zones or sections of the furnace named to describe the process that occurs in each zone of the furnace. Generally, the first zone of a travelling grate furnace is the updraft-drying zone. The present invention is used in this section of the furnace, as well as the next zone called the downdraft-drying zone (DDZ).
As an example, consider that a hearth layer of fired pellets 3 inches deep is placed upon the pallet grate bars. A layer of finished pellets 15 inches deep is then placed upon the hearth layer, making a total depth of 18 inches. The hearth layer is dry and the pellets in the finished pellet layer contain 10% water. The grate bars are aligned on the pallet to provide openings about ¼ inch wide to permit hot air to flow through the openings.
The updraft-drying zone of the furnace consist of windboxes beneath the travelling grates. Each windbox is designed to provide a reasonably airtight seal to force air under pressure up through the bed of pellets that is on the travelling grate.
A large quantity of air is directed up through both the hearth layer and the layer of finished pellets. The air temperature is generally 600° F. to 850° F. This description applies to a continuously travelling grate machine that is in equilibrium for temperature and airflow. As an example, consider an 8-ft. wide by 8-ft. long windbox. Assuming the grates travel 96 inches a minute, any pellets are above a windbox for one minute. Hot air is forced up through the pellet bed by a forced draft fan. Sufficient upward velocity and static pressure is maintained to establish an upward airflow. The hot air blowing by the finished pellets evaporates surface water while water inside the pellets slowly evaporates. Some of the heat energy warms the pellets, but most of the heat is used to evaporate water on and within the pellets. The heating and evaporation proceeds from the bottom up through the pellet bed. The transfer of heat travels slowly up through the pellet bed. The evaporation of water cools the air by an amount of energy called the heat of vaporization. The heat transferred to solid masses such as the pallet frames and the hearth layer is called sensible heat transfer.
It is necessary to understand some of these physical changes to evaluate the potential attributes of my invention. Moist air travelling up through a bed of cold pellets is eventually cooled to the dewpoint temperature so that water vapor condenses on the cool pellets, thereby increasing the water co
Esquivel Denise L.
Harmon James V.
Shulman Mark
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