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THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 14, No. 7
Manufacture of Phosphoric Acid in t h e Electric Furnace by t h e a-n ric Precipitation Method' By Theodore S w a m 2 BROW~MAR BUILDING, X BIRMINGHAM, ALABAMA
The commercial production of phosphoric acid i n the electric furnace by condensation and electrical precipitation is now being successfully conducted. Methods of purification haw been worked out and are now in operation. Processes have been developed and are actually operating f o r the manufacture of various high-grade phosphate products.
HOSPHORIC acid is one of the principal mineral acids used in agriculture and the industries. By far the largest portion is used in the fertilizer industry, but there has been a decided increase in its use for food products and numerous industrial purposes. The method which has heretofore been generally used consists in treating a phosphate, usually bone or phosphate rock, with sulfuric acid. When the phosphoric acid has been converted into a soluble form, it is separated from the residue. Acid produced in this way is too dilute for many purposes and must be concentrated.
P
One of the outstanding features of the electric furnace condensation and precipitation method is the production of high-grade concentrated acid from phosphate rock containing a high percentage of iron. The process is new and differs from others which have been proposed in that it produces directly a high-grade concentrated acid and ferrophosphorus. Phosphate rock containing a high percentage of iron is not) objectionable to this process, because the iron contained in the rock as well as that in the iron borings is removed as ferrophosphorus. Thus, an acid is produced which contains less iron than acid produced by the sulfuric acid method, even when the latter uses a phosphate rock containing a very low percentage of iron.
PURITY OF PRODUCT The acid shipped is water-white, complies with all Federal and State Pure Food Laws, and is of approximately the following analysis :
ELECTRICAL PRECIPITdTION M E T H O D Three furnaces are in full operation a t Anniston, utilizing about 10,000 h. p. Electric power is supplied by the Alabama Power Company from its hydroelectric plant on the Coosa River and is delivered to the furnace transformers at 44,000 volts. Provision is made for varying the voltage in the furnace, as operating conditions may require. Phosphate rock is obtained from Tennessee, though some Florida rock has been used; coke comes from the Birmingham district; and sand is available from local sources. The process consists in charging a mixture of phosphate rock, coke, sand, and iron borings into an electric furnace, and smelting the charge so as to liberate the phosphorus in the phosphate rock. The iron absorbs a certain portion of the phosphorus and forms ferrophosphorus, which is tapped from the furnace. The lime and sand combine to form a slag, which is also tapped from the furnace. The ferrophosphorus analyzes from 24 to 26 per cent phosphorus, although the analysis may be varied depending on the grade desired. The phosphorus not absorbed by the iron passes through the charge together with the other furnace gases, which are oxidized by air and drawn out of the furnace into the condensing and precipitating apparatus. The acid as collected analyzes from 90 to 95 per cent &Pod, is a light brown color, and has practically no suspended solids. A typical analysis is as follows: Hap04
Per cent BO t o 95
n ne
The acid from the condenser and precipitator is treated for the removal of certain objectionable impurities. Special provisions must be made to prevent acid of this strength from crystallizing. 1 Address presented before General Meeting at t h e 63rd Meeting of the American Chemical Society, Birmingham, Ala., April 3 t o 7, 1922. * President, Federal Phosphorus Co.
Hap04
Fe FePOa CaO $003
Pb
AsaOa
+ AlPOa
Per 75 0 0 0 0
cent
05 30 01 10 0 009 P p m 8 0 5
Shipments are made in tank cars, barrels, and carboys. Tank car shipments have proved satisfactory. A wooden tank car is lined with a special wax compound that does not discolor the acid or affect its composition. Barrels are also lined with this compound. Purified acid of this grade is used in producing high-grade salts for use in medicine. It is unnecessary to recrystallize to make a salt of exceptionally high purity. For instance, phosphate of soda has been made of a higher purity than any medicinal phosphate. An acid of unusual purity can be prepared by crystallizing the acid from the condenser and precipitator. A U. S. P. 90 per cent has been made which, besides being suitable for all drug and pharmaceutical uses, is also adapted for the manufacture of hydrogen peroxide. Construction is now going on for a plant to produce high-grade acid in this manner. Purified acid of 7%5per cent grade is being regularly shipped as liquid phosphoric acid and is being used in the manufacture of phosphoric acid paste and crude. monocalcium phosphate, for sugar refineries; in trisodium phosphate, for water softeners; in phosphate of soda, for textile uses. Equipment is being installed to make U. S. P. phosphate of soda, both anhydrous and crystals, monosodium phosphate, ammonium phosphate, and concentrated fertilizers. APPLICATIONTO FERTILIZER INDUSTRY To obtain the maximum value of the developments which have just been described, this process must be applied to fertilizer production. The record of plant operation a t Anniston during the past three years clearly indicates that with cheap hydroelectric power and proper plant location, phos-
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