ping dry caustic is in 400-pound drums, although some users get larger lots shipped in "tote bins." These tote bins are built by the user and are shipped by rail on flat cars. Caustic producers have long been searching for alternate shipping methods, but the hygroscopic nature of the product has been a complicating factor. Diamond actually began using part of the "Source-to-Silo" system last No vember as part of its continuous dry caustic process. After processing, the caustic is transported by a dry air con veyor system from the flaker, through a mill, onto screens, and into storage. Up to now, however, conventional drum shipment was used. Diamond worked with Union Tank Car to find a way to handle bulk an hydrous caustic by rail car. Minor changes in piping and hopper design of Union's 2800-cubic-foot pressureflow cars allow dryness specifications to be met during loading and unload ing; these cars were used for field tri als. New 3500-cubic-foot, 90-ton cars are on order from Union and will be used when commercial shipments be gin. At the customer's plant, the bulk caustic is unloaded by either a blower or an air compressor through a flexible hose into a predried silo. Unloading is done at 1000 pounds per minute, compared with about 200 pounds per minute for drum-shipped caustic. The caustic can be transferred from the storage silo via a hooded weighing hopper direct to the user's process. Vent systems prevent atmospheric moisture from entering the silo during transfer.
LiC03—role in aluminum making? The cut in the price of lithium carbon ate from 45 to 3 8 1 / 2 cents a pound (C&EN, May 16, page 25) has prompted aluminum producers to again consider using the electrolyte in their potlines. Lithium carbonate, which increases conductivity in the electrolysis cell, could raise potline ef ficiencies as much as 15%. Thus, primary aluminum capacity could be increased without adding more potlines at a time when alumi num makers are scrambling to meet burgeoning demand for the light metal. Shipments of the metal in 1966 might rise more than 10% above the 1965 total (C&EN, May 2, page 27). The high cost of lithium carbonate has prevented its use in potlines be fore. Only time and further study by aluminum producers will tell whether the lower price will justify its use now. Although aluminum capacities could be increased by using lithium carbon 24 C&EN MAY 23, 1966
ate, no U.S. aluminum producer is us ing the chemical in a commercial proc ess. In the Hall process used to make aluminum, molten metal is produced by electrolysis of alumina in a molten bath of cryolite (a sodium-aluminum fluoride), calcium fluoride, and alu minum fluoride. The electrical energy fed to the cell converts the alumina to aluminum and oxygen and also keeps the bath molten. The molten alu minum settles to the bottom of the cell where it is drawn off. Aluminum output of the cell could be increased by increasing the current fed to the cell. But a point is reached where the increased current is offset by voltage losses due to the concomi tant increase in temperature of the electrolyte. Moreover, aluminum tends to oxidize at higher bath temper atures. If lithium carbonate is added to the bath, though, current density can be increased about 5% with no increase in bath temperature. In a patent (U.S. 3,034,972) issued in 1962, Kaiser Aluminum and Chemical says that current densities could eventually be increased by 25% by adding lith ium carbonate.
Three routes to sulfur removal The debate on the cost versus the ben efit of lowering the sulfur content of coal got a new reference point last week in Columbus, Ohio, in the form of a cost study by Raymond E. Zim merman of Paul Weir Co., Chicago. The study covers three coal prepara tion plans (A, B, and C—see table) designed to remove maximum sulfur using the most efficient processes. In each case, combined mining and operating costs work out to about 15 cents per million B.t.u. But many un known factors in plans Β and C must be evaluated. The ultimate objective is to reduce sulfur dioxide emission from burning coal, particularly in electric generat ing plants (see page 5 6 ) . In 1965, such plants took 55% of the 450 mil lion tons of coal consumed in the U.S., Mr. Zimmerman said at the national meeting of the American Institute of Chemical Engineers. Coal preparation plants are based on pulverizing and washing. They remove only pyritic sulfur, Mr. Zim merman points out. Organic sulfur, that combined chemically with coal hydrocarbons, can be removed only by changing the coal to liquid or gas and often is relatively high—in one case more than 2% in a total sulfur content of 3.1 to 6.7%. Currently, Mr. Zimmerman says, much of the electric utility coal isn't
cleaned at all. Some is partially cleaned to remove rock. The com plete preparation plants that wash utility coal separate coal that has a specific gravity of about 1.60. Lower specific gravities (about 1.30 for pure coal) result generally in lower sulfur and ash content. But yield is also lower, resulting in higher costs. Mr. Zimmerman's cost estimates give the relative costs for three sepa rate approaches to coal preparation. In the first, plan A, coal is crushed near the mine to 3 / 8 or V 4 inch—the smallest particles feasible for transpor tation in open railroad cars. Plan Β grinds to 200 mesh, with final grind ing near the power plant. The third approach, plan C, gives a 14-mesh product in slurry form that can be pumped through a pipeline and burned wet in cyclone burners. This
About 15 cents per million B.t.u. (Based on plant capacities of 1000 tons per hour of coal processed; base mine cost of $2.45 per ton of run-of-mine coal)
Item
Plan A
Plan Β
Plan C
Capital cost $5.6 $6.3 $5.1 million million million Mining cost $3.141 $3.043 $3.141 (per ton) Operating $0.370 cost(per ton)
$0.490
$0.310
Dépréciation (per ton)
$0.117
$0.142
$0.106
Total
$3.628
$3.675
$3.557
Cost perl
15.1
15.3
14.8
million B.t.u., at
cents
cents
cents
12,000 B.t.u. per pound Source:
Paul Weir Co., Chicago
plan takes advantage of Consolidation Coal's experience with its former slurry pipeline and in wet burning. Plan A involves crushing, rock removal in a Baum jig, and screening of coal, followed by cleaning in dense media cyclones at, possibly, a specific gravity of 1.35. Float material is clean coal. Sink material is further ground, treated by hydrocyclones to remove a high sulfur reject, treated by froth flotation, then dried. In plan B, the 3 / 8 -inch coal (following primary treatment in a Baum jig) is sent to the power plant. There it is pulverized and separated in air classifiers. Material passing 200 mesh goes directly to power plant burners. The remainder is treated by hydrocyclones and froth flotation as in plan A. Plan C follows the operation in plan A. Instead of drying, however, the product is pulverized further to —14 mesh and slurried for pumping.
