I/EC Guidebook for Technical Management
EQUIPMENT AND DESIGN
A Direct Air-Quenching Cooler Without increasing actual mining operations, Aluminum Co. of America boosts a l u m i n a yield. A highly specialized process reclaims this precious powder from refuse mud from normal Bayer plant operations A Staff Feature
•VEFINING of alumina from bauxite ore is an intricate process, complicated by the abrasive, sticky nature of the ore and the various chemical and mechanical treatments required. This means that the greatest yield possible is necessary from each pit. At the Bauxite, Ark., works of Aluminum Co. of America, mining is simultaneous in several different open pits. This multiple mining permits blending of the raw material to conserve the high grade deposits. Material is carried directly to the mill by trucks which dump directly to process. Actual processing consists first of preparing the ore. I t is crushed in breaker plate hammer mills, and then fine ground in rod mills. Conversion of this ore into the basic alumina combines both chemical and mechanical procedures, centering around specialized filtering operations in the Bayer plant.
Waste from Bayer Plant Meant Lost Material
In the course of filtering operations at the Bayer plant, a large amount of refuse mud from the ore develops. This mud contains a small amount of alumina oxide, and at one time was disposed of as waste. Alcoa officials deplored this waste, but realized that a highly efficient reclaiming process would be necessary to extract from the mud its small
General view of part of Alcoa Plant in Bauxite, Ark.
quantity of pure alumina. I n creased demand for this product meant an even more critical examination of the problem, resulting in an entire new system which has resulted in substantially increased recovery. This new system is, in broad outline, similar to the original refining operation. From disposal at the Bayer plant the sludge is put through remixing, to increase the caustic content. Then, in this slurried form, it
is subject to the sintering action of a giant rotary kiln. Direct Air-Quenching Cooler Speeds Process
Material in the kilns is raised to a temperature of 1800° F., to produce the sinter. To cool large quantities of this material enough so that it could progress to the next refining phase was a major problem. This was solved with the installation of direct, air-quenching inclined-grate V O L 5 1 , NO. 12 ·
DECEMBER 1959
63 A
I/EC
Guidebook for Technical Management—EQUIPMENT AND DESIGN
Series of r o t a r y kilns brings material up to 1 8 0 0 ° in a sintering process which recovers alumina from the refuse r e d mud
coolers made by Fuller Co., Catasauqua, Pa. The sinter clinker from the rotary kiln drops by gravity through a vertical throat onto the grates in the coolers. As these grates urge the material forward over 32 feet of grating, cool air is brought up through the grates to surround the clinker. Fines falling through the grates are moved by drag chain to discharge out the back end. These coolers, designed to handle 18 tons an hour, bring the temperature of the clinker from 1800° down to 150° before discharge into an en-
Fuller control panel for cooler also keeps record of v a r i ables such as t e m p e r a t u r e a n d speed
closed oscillating conveyor. A hammer mill at the discharge end further crushes the sinter. A particular labor-saving with this kind of cooler, pointed out by operating personnel, is the fact that it will take all of the product from the rotary kiln. This includes sinter which has developed a fusion ring formation (hot zone). With other types of cooling equipment these fusion rings had to be scalped out individually. Two of these coolers are now in service at the plant. Operation of each cooler is controlled from a Fuller-designed and engineered control panel, which not only contains all necessary stopping, starting, and visual indication components, but also makes a permanent record of such variables as temperature and speed. The oscillating conveyor which receives material from the discharge ends of the coolers delivers to a bucket elevator leading to storage bins. Further steps in refining of the alumina include additional grinding and leaching operations.
coolers in place of rotary-type indirect air coolers. Bed of the cooler is at a 10° slope. Stationary grates alternate with movable grates which propel the material at a uniform rate throughout. Cooling air is blown by a forced draft fan into the open space beneath the grate. This air chamber is divided into compartments and equipped with louvre dampers for even air distribution. Air is forced through the spaces between the grates, and through the openings in each grate. Constant movement of the movable grates continuously changes the position of the clinker as it is tumbled from grate to grate, thus exposing all surfaces to flow of air from two directions. A porous material bed, through which the air may pass in contact with each particle, is maintained throughout. Maximum recovery of heat is attained by directing all of the air required for combustion through the upper or recuperator section of the grate. In this way, fuel consumption is reduced.
Cooler's Design Brings A d d e d Benefits
Inclined-grate cooler with b r e a k e r (center background) discharges sinter, cooled from 1 8 0 0 ° to 1 5 0 ° at the r a t e o f 1 8 tons an hour, into enclosed oscillating conveyor (foreground) 64 A
The rate of 18 tons of material cooled an hour represents an increase in production of between 2 and 3 tons for each cooler each hour. Officials at Alcoa also report maintenance costs have been reduced by using these direct air-quenching
INDUSTRIAL AND ENGINEERING CHEMISTRY
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