Olt SUIIIC h i e t lie luct has been known that fluorine can be volatilized from rock phosphate and its phosphorus content thereby rendered available for plant food by heating the rock in the presence of water vapor and silica (12). The chemical reartion involved in the defluorination of rock phosphate presumably may be reprefiented by (he following cyuation:
F
' l hc1efluorine.ted product consists princilially of tricalcium phosphate; the> Buorinc. is volatilized as hydrogcn fluoride. Jaroh and co-iTorkers studied the [lefluorination of rock phosphate on a laborntory scale (18, 14, 15, 1 6 ) ; their work was iiirected toxvard developing a process in which the defluorination would be accomplished by calcination-that is, by heating at a temperature somewhat below the fusion temperature of the rock phosphate. Their results sl-iom-ed that, whcn thin layers of rock phosphate containing 4 to 127" silica were heated in the presence of water vapor at 1400" C. (2552" F.) for 30 minutes, 95% or more of the fluorine [vas volatilized; the prodnrt vas sintered but not fused. The developmeid of a defluorination process vas undertaken by TVA since i? appeared possiblc that phosphate fertilizer could be produced by defluorination a t i i lower cost t,lian by the commercially used acid process. Many attenipta \rere made to carry out the defluorination of phospliatr rork by calcination on an enginccriiig s c a l ~ . by TT'A (3)and others, Several difficulties xeTe encountered in attempts to carry out the calcination process in a rotary kiln, Control of tho temperatures in thc narrow range just b c l o ~thc fusion temperature wa$ dificult; sintering of the phosphate into a pasty mass caused mechanical difficulties ani! interfered with contact betxecn the gas and solid to such extent t,liat the dcgrcc of defluorination \vas not satisfactory; attack of the refrnctoricls of thc kiln lining was
severe.
1208
T. P. H
G N ETT
A rlD
T. N. HUBBUCH TENNESSEE V A L L E Y A U T H O R I T Y .
W I L S O N D A M , ALA.
,
Because of these difficulties, TVA abandoned the calcination process and began an investigation of processes involving fusion of rock phosphate and defluorination of the molten material. Laboratory scale studies (8) showed that the rate of defluorination increased as the fluidity of the, melt increased, increased as the water-vapor content and velocity of the furnace atmosphere increased, and decreased as the depth of molten material increased. The fluidity of the melt was affected by its chemical composition (7) and temperature. Under favorable conditions 90% or more of the fluorine could be volatilized in 10 minutes. Since it was found (8, 11) that tricalcium phosphate was the chief constituent of the defluorinated product, the term "fused tricalcium phosphate" was adopted to designate the product obtained by fusion and defluorination of rock phosphate. Furnaces of many types were tried (3) for fusion and defluorination of phosphate rock. Most of them were unsuccessful because of low thermal efficiency,short furnace life, mechanical difficulties, or inadequate defluorination. A more promising process, developed on a pilot-plant scale, consisted in melting the rock phosphate in a direct-arc single-electrode furnace, tapping the molten rock into a shallowpool oil-fired hearth furnace in which defluorination took place by reaction with the water vapor content of the furnace atmosphere, and quenching the molten rack as i t was tapped from the furnace. The temperature of the molten rock ip the hearth furnace was held a t about 1550" C. (2822' F.), the depth of the pool was about 2 inches, and the time required to defluorinate the melt to 0.2% fluorine was 80 minutes. The production of 1 ton of fused tricalcium phosphate containing about 30% PzO;and 0.27, fluorine in the pilot plant (10 tons per day capacity) required the use of 700 kw.-hr. and 12 pounds of graphite electrode in the melting furnace and about 110 gallong of fuel oil in the defluorinating furnace. The fuel oil requirement probably could be considerably reduced by use of larger scale equipment, and by recovery of a part of the heat in the stack gas for preheat-
.
1209
1210
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 3.
WATER
A
Vol. 38, No, 12
Demonstration Plant for Production of Fused Tricalcium Phosphqte
OVERFLOW TO QUENCH WATER RESERVOIR
PA
SHIPPING
ing the combustion air. However, decision on the further development of this process was reserved pending the outcome of experiments on the fuel-fired shaft-furnace pilot plant which were then in progress and which eventually resulted in a more economically promising procesd. The use of a shaft furnace for melting and defluorinating phosphate rock appeared promising because the countercurrent movement of the ascending hot gases and the descending charge would favor relatively high thermal efficiency. Also it was believed that the material as it melted in the furnace shaft would form thin films and droplets, which would provide intimate contact between the gas and molten material and thereby facilitate rapid defluorination of the molten material. PILOT-PLANT DEVELOPMENT
The pilot-plant furnace, shown in the photograph on page 1208, had a capacity of about 10 tons of fused tricalcium phosphate per day. The cylindrical, refractory-lined shaft was 5 feet in diameter and 15 feet high. The top of the furnace was enclosed with an unlined steel hood containing access doors through which the charge was inspected and rodded if necessary. The furnace was fired with gas or oil by burners located near the bottom of the shaft and was charged with a skip hoist. The charge consisted of phosphate rock and silica in such proportions that the product contained 20 t o 25Y0 silica. In some cases the phosphate contained enough silica so that none had to be added. The melt was tapped from the furnace at regular intervals, usually 40 minutes, and was quenched with high-velocity water jets in a trough; this treatment granulated the product to a sand-size material. When the furnace was fired with gas and no steam was added, the fused product contained about 0.5% fluorine'; the water vapor content of the furnace atmosphere was about 47,. This
concentration of water vapor resulted from combustion of minor percentages of hydrogen and hydrocarbons that were present in the carbon monoxide gas and from small concentrations of water vapor in the combustion air and gas. When enough stcam was added t o the burner to raise the concentration of xater vapor in the furnace atmosphere to about 12%, the fluorine content of the product decreased t o an average of 0.2%. A permissible maximum of 0.4% fluorine was t,entatively established at this time through greenhouse tests of fused products from previous experimental work (If). Subsequent greenhouse and field plot tests indicated that material of this fluorine content is acceptable, and the specification of 0.4yG maximum fluorine content has been retained. Since the experiments Kith the gas-fired furnace shovied thbt the presence of as much as 12V0 of water vapor in the furnace atmosphere TYas desirable for obtaining an adequate degree of defluorination and since the combustion of fuel oil would give a furnace atmosphere containing about 14% water vapor, the pilot plant was changed to provide for the use of oil as fuel. Tke fuscd phosphate produced in the oil-fired pilot plant had fluorine contents that ranged from 0.15 to 0.367,. Natural gas would give a water vapor concentration of about 20%, which would have been still more favorable for defluorination, but it, was nr t available. Extensive tests were made in the pilot plant with the followii g kinds of phosphatic charge materials: Sinter prepared from a mixturc of Tennessee phosphate matrix (raw phosphate as mined) and washed sands, nodiilized Tennessee phosphate sand, ex1-411 0uorine determinations reported in this paper were made by the widely used Willard and Winter method. A recent study of this method in the T V A laboratories indicates that it gives somewhat less than the true fluorine values for fused tricalcium phosphate.
.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December, 1946
truded pellets of Tennessee sand, TenTable 1. Composition and Particle Size of Phosphatic Charge Materials nessee blue rock, Florida pebble phosfor Pilot-Plant Furnace phate, and unagglomerated washed Moisture Tennessee sand. Table I gives the 'Ontent a8 Chemioal Composition, % (Dry Baais) . chemical composition and particle size Particle Charged, Ignitioa of these materials, Table I1 presents Material She, In. % PsOr CaO SiOa Fer01 Ala01 F loss furnace operating data. Sintered sand-matrix mixt.5 2 23.8 33.8 24.3 6.6 7.4 1.9 ... Nodulized sand' 0 31.6 44.7 14.0 4.6 2.4 4.6 Sintered phosphate was produced :Extruded sand pellets" 1b 2 8 . 2 38.7 18.9 3.2 2:s 2.9 2.9 Blue rock' 28.6 41.1 13.5 2 2.3 3.1 4.3 in a plant-scale, downdraft sintering 4.7 Florida pebble 1 31.9 46.4 6.8 7.0 2.0 3.6 1.4 machine, and nodulized phosphate Unagglomerated aanda 16 26.3 3 6 . 1 20.8 3.3 6.1 3.3 2.8 sand was prepared in plant-scale rotary 0 Tennessee materials. kilns as described elsewhere (4). Extruded pellets were prepared from Tennessee phosphate sands in experiTable II. Typical Operating Results of Pilot Plant5 mental equipment by mixing and Silica b Fuel Added Consumption Production grinding the wet sands in an edge Lb./100 Lb, Million B.t.u.7 Rate, Tons/ Compn. Of Produoti ?% Charge material Phosphate Tonof Product 24 HI. PzOr Si01 Rt01 F runner, adjusting the moisture content to about 18% by adding water Carbon Monoxide Gas (286 B.t.u./Cu. Ft.) an Fuel gr dried sands, and extruding the mix8 8.6 10.8 29.8 20.0 ... 0.52 Blue rook Blue rock$ 8 12.2 8.7 30.3 20.0 ... 0.20 ture in a standard deairing auger Oil (138,000 B.t.u./gal.) a8 Fuel machine through dies having openings Sintered sand-matrix mixt. None 7.9 B5.2 24.4 13.9 0 . 3 8 11.1 to 8/4 inch in diameter. The exNoduliied sand 15 9.9 10.2 29.6 20.6 9 . 0 0.32 truded columns leaving the machine Extruded sand pellets None 11.0 9.0 30.6 19.6 6 . 6 0.31 Blue rock 11 11.3 7 . 1 2 9 . 4 20.4 b . 8 0.17 broke into random-length pellets and Florida pebble 20 b.0 0.16 16.3 7.8 28.7 22.9 Unagglomerated sand None 17.1 27.9 2 3 . 2 11.6 0.16 6.0 were used as furnace charge in the moist condition. Usually two grades 5 Air not preheated. b Sand or gravel containing 88 to 95,% Sios. of phosphate sand were blended in e Steam was added at the burner to increase the water vauor content of the combustion zaa to about preparing mixtures for extrusion so 12%. that the resultant mixture had the proper silica content. The blue rock was prepared as furnace charge by crushing and screening. No preparatory treatment was used perature of the preheated air a t the burner inlet was varied for Florida pebble phosphate or unagglomerated Tennessee phosfrom 500" to 1700" F. The amount of heat Cequired per ton of phate sand. product was found to be about the same, regardless of whether all It was possible to fuse and defluorinate each of the charge maof the heat was supplied b y combustion of oil or whether part of terials; however, the w e of unagglomerated washed sands ( - I / , the heat was supplied as preheat'in the combustion air. Since inch) resulted i.1 unsatisfactory furnace operation, about 30 to the stack-gas temperature usually was about 1000° F., it is pos40% of the sand charged was blown out of the furnace as dust, sible that the heat in the stack gas could be used for preheating tLnd the fuel requirement per ton of fused product was high. The the combustion air with a resultant saving in fuel cost. The economic advantage of the recovery of the stack-gas heat would dust loss for other burdens varied from 5y0 for Florida pebble to a aegligible amount for the blue rock. depend on-a balance between the fuel saving and the investment The heat requirement of the charges other than unagglomerated cost and operating expense of the necessary heat-recovery equipsand increased from 8 to 15 million B.t.u. as the Rz03 (iron and ment. Since the economic advantage appeared doubtful, no aluminum oxide) content of product decreased from 14 to 590 provision was made for recovery of stack-gas heat in the large (Table 11),probably because the fusion temperature of the charge scde plant. increased as the RtOa content decreased. For example, Florida In several tests fused tricalcium phosphate was remelted and pebble phosphate containing 3.49* RnOa had a fusion temperature more completely defluorinated in the pilot-plant furnace. The of 2880" F., and sinter containing 13.1 per cent Ry03 had a fusion remelted product contained less than 0.10% fluorine and was temperature of 2580' F. The heat requirement for Florida pebused for animal feeding tests. When the granulated product was ble charges probably could have been decreased by adding a flux remelted, 20 to 30% was lost from the furnace as dust; when unmaterial containing iron oxide and/or alumina in appropriate quenched lump product was remelted, the dust loss was negligible. proportions. The fluorine content of the product decreased The fuel required for remelting previously fused material wss from 0.36to 0.15% as the fuel consumption increased from 8 to about 9 million B.t.u. per ton of product. 15 million B.t.u. ; the more complete defluorination obtained DEMONSTRATION PLANT with the higher fuel consumption was attributed to the increased quantity of water vapor that resulted from combustion of more Since cost estimates based on pilot-plant data indicated that fuel oil per ton of product. fused tricalcium phosphate could be produced a t a low cost and Furnace operation was easiest and fuel consumption was lowest since extensive field tests indicated that the product was about as when the charge consisted of sinter. prepwed from a mixture of beneficial t o plant growth as concentrated superphosphate per washed phosphate sand and matrix. The sintered and noduunit of total P ~ Oapplied, S a demonstration plant was built t o lized charges contained less fluorine than the raw charges, obtain more complete information regarding the production of since about 3001, of the fluorine originally present in the raw fused tricalcium phosphate and its use as a fertilizer material, phosphate had been volatilized by the sintering or nodulizing The plant was constructed near Columbia, Tenn., a t TVA's process. However, this lower initial fluorine content had no phosphate mining and! concentrating plant. Since the plant waa perceptible effect on the fluorine content of the fused product. constructed during wartime, it was necessary to use as much salTests were made to determine the effect of using preheated and vaged material and surplus equipment from other TVA plants unpreheated combustion air on furnace operation with gas and as possible. Figure 1 shows the two furnaces and the storage with oil as fuel. The air was preheated in pebble-filled regenera#pileof furnace product, and Figure 2, the product drying and tive stoves, which were fired with carbon monoxide gas. The temshipping facilities. The design capacity of the plant was 120
-
,
1211
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1212
ton. 01' product per (lay. Tlik rapacity was considered t o be about the minimum scale that xoulcl provide concliisivc data regarding the economic possibilit ies of the process. Tile plant \vas started in Juiir, 1:145. Ihriny t l i v first yc:ir pacloal improvement has Iieen lirunglii about by piirferting operating techniques and cliwnyiiig iquipment . .i .satisfartory furnace design was devr~lopt~tl I)? cui ivc. c.li:tngc~swliicli prccluded siniultsneous oi)i'i,a.Tiriii()I' tii iii~ii:ires tliuiiig Imrt i l l the year; for this rwsoii all iJpc~t~:iiilip tl:it:i i i i this p : t p on a one-furnace basis. iiii,e, ; I I < , t ' i i i ~ n : i ~ ~:IW t ~ ~ i i l i , i i t i i x l , ilii.!,
CHARGE
t h
USUAL CHARGE LEVEL I
1
+-WATER
SPRAY
COOLING COILS
BOqH
LUMNITE CEMENT A N 0 GROG
5
HEAR,TU 6
TAPPING SLOT SPRAY- WATER CATCH 8 A S I N
Vol. 38, No. 12
+
niiniinalparticle size of t l w t J r i J \ V i l ruck wah 1; - 3 inc.1it.h. thix sinter was - 6 iixhes; ho\~cvc~r, both ~naterial-. ~ ~ ~ i i i ~an : iappreciable iii~d amuuiit of fincs tlnv t o impcJrfcrt srrt'l-i1iiig or iivgradation in handling. l;iyiiri* 3 is a siniplifid flow c1iagr;iin u l the iris:il trivali4iiiii 1)liiiy)Iiiitc plant, and Figure i shows one of the furnacw. b'iirii:i1,11 vliarye materials ~ c Imuglit r ~ with trucks t o the r:tx iii:ti t , i i ; i l * tiuililing, nliich liad :I shorage capacity of a,bout 500 t i ) : i h . ., I t i t ' i,li:irge nnterinls wvre transferred xitli an overhead Iiu~~kt*t i'i~:iiit' 1 i i ii lioppcr (1% cubic foot capacity) inonnted ovi'r i t i t , - k i p [lit. IVIitw tlic f i i i m w clinrge consisted of two or 1iiori' iii:iI c,r.i:il-.thn. iixiterials i v t ~ c proportioned . to the charge hoppi.1. voiiiiiic.1 riixlly liy tlic cr>ii~c> Iiucketful. The t1csirc:tl q u a n t i t y ( ~ i . ~ ~ l i a t \\-:is y i ~ 11r:iwn from T lic bottom of tlio rliargc lioppcr i i i l i l :I \vi,igii Iio1ip~r,\veighed. :inti ~lunipedinto t l i ~skip, , \vliit.li t i ' i ~ i i h ~ I I I ~ I [ Y1 Il i e > c,li:irge to t h rtw4virig lioppw ut tiit, top oi' t h iui,iiac,i.. 'I% iihiial \wight of :L \kip load was 1 ton. Thc ~:Ixirycw i h ~ l i m i p t ~froni d the rcc-c-iviiighqiper into the Iurri:ice by o p i ~ i i i i i #, I 11t,11 :it itie bottom of the lioppcr. The bell was oiw1:iitvI liy :I t l air eglintlcr froni cuiitrols located at t lie fiiix:ire : i " i ~ w (1;igure ij. .41i opcrator, stationed :it t rniined by inrpcction when the furnacc r ing and informed the skip filler by means of a signal vliarge level was usually ninini ainetl il i p arwss doors. Tlic charge did n u t ile nil uiiiforrnly aiid riiiiiiiiuody i t i I / I O sliaft of thc furnace; usually crusts formed :it tlit: t i i p (11 l l i i . charge column and fell in a t intervals as was the case Xvitli tlic. pilot plant furnace. However, the charge fell in a t much niorcs Irequent intervals in the large furnace; as a result, chargia \vtih iiddeti inore regularly. With favorable charge iuaterials siii.ii :I\ Iiron-n rock and sinter cont,ainingonly a small proportion of f i l i i 3 5 111' mud balls, usually some charge could be added each Iroiii,: iiitervals of more than 3 liours tluring Tyhirh no charge c*iitiliilii, :iilded were rare. Generally a ring of sintercd vharge adhered t i J I lie brirhwork iii i he top of the shaft and left an opening, called a "chimn varying diameter (usually about 3 feet) through which ~ l i ccliargc~ iltwended and the combustion gases ascended. Sincia ilii' gab wlocity a t this point was high, a considerable amount, of tlusi \vs, t i l o w i out, of the charge. Most of this dust settled bark onlei t l i t , Iharge column as the gas passed through the hood where ilic g:is velority \ ~ n *relatively low; the settled dust evi:ntually beimnc i~nirainrili n the charge reaching the fusion miv. Only al)oril .>c; (IF t I i c , i,liarge was lost, from the furnncc Tlii* :ciril
+
I
SECTION A-A
Figure 4. Large Scale Furnace for Production of Fused Tricalcium Phosphate
Table I l l . Composition of Brown Rock and Sinter Charged to Large Fusion Furnaces
December, 1946
'
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1213
12 pounds per square inch by electropneumatic control of the comTable IV. Typical Screen Analyses of Fused Tricalcium pressor unloading valves. Phosphate Each furnace was originally equipped with four burners (FigCumulative Per Cent Retained on ures 4 and 5) which had individual air and oil metering and control U. S. Screen Size systems. The oil flow t o each burner was maintained at a con6 mesh 16 mesh 40 mesh 80 mesh 140 mesh stant rate by manual control and was indicated by a rotameter. Granulated furnace prod7 22 66 94 uct after drying The air flow to each burner was indicated by a maqometer that Dried product after .. 11 57 92 98 showed the pressure differential across a standard-type orifice screening -10 mesh and grinding oversize meter and was controlled by a valve located near the burner. Dried product after 14 45 62 grinding in ball mill The air control valve was operated by a hydraulic cylinder from the control house either manually or automatically to provide the desired air-oil ratio; this ratio varied from 1275 t o 1650 cubic feet of air (measured at 60" and 30 inches of mercury) per gallon cal screen analyses of the granulated furnace product and of of oil (measured at 60" F.), depending on the oil composition and products prepared for shipment. moisture content of the air. The air pressure a t the burner usually was 3 to 10 pounds per square inch, depending on the air flow rate and the pressure in the furnace. IMPORTANT FACTORS IN FURNACE OPERATION Molten, defluorinated phosphate rock accumulated in the It was found that accurate control of the air-oil ratio was eshearth of the furnace and normally was tapped a t intervals of sential t o good furnace operation, probably because the air-oil about 1 hour. The temperature of the melt as tapped from the ratio significantly affects the flame temperature. The best refurnace was 2550' to 2700" F. The amount of melt in the fursults were obtained when the air supply was about 97 t o 100% nace just before tapping was such that the burner-port outlets of that stoichiometrically required for complete combustion of were submerged and the flame from the burners bubbled through the oil; under these conditions Orsat analysis of the combustion the melt. Submergence of the burners by the melt resulted in gas sampled at the top of the charge column showed 15.5%carbon rapid fluctuations in the pressure a t the burner, as indicated by a dioxide, 0.5% carbon monoxide, and 0.0% oxygen. It was also pressure gage in the control house; this pressure fluctuation was noted that the melt was easier t o t a p and was more fluid when the criterion by which the operators judged whether there was the air-oil ratio was in the range of 97 to 100% of stoichiometric. sufficient melt in the furnace for tapping. The melt was tapped Considerable experimentation was necessary to develop a satinto a brick-lined concrete trough about 2 feet wide and 2 feet isfactory burner-port design and t o determine the optimum numdeep, where it was quenched and granulated with high-velocity ber of burners and distribution of oil among the burners. The water jets. About 1500 gallons of quench water per minute were furnace was designed t o operate a t a total oil-burning rate of 150 supplied at 100 pounds per square inch pressure through seventeen gallons per hour and was originally equipped with four burners, jets having a total discharge area of 6 square inches. The quench each having a n oil-burning capacity of 25 to 60 gallons per hour. water containing the granulated product discharged into a pit Each burner was installed at the end of a cylindrical burner port (approximately 2000 cubic foot capacity) where the granulated (combustion chamber), 30 inches in diameter and 50 inches long material settled out. The water overflowed from the quench pit (inside dimensions of steel shell), which connected with the hearth to a reservoir from which it was recycled. The granulated prodof the furnace at a 30" downward angle. The burner ports werc uct was dipped out of the quench pit with a bucket crane and originally lined with mullite refractory t o an inside diameter of piled in a concrete-paved storage area, which had a capacity of 21 inches (Figure 5); however, this refractory was melted away about 5000 tons. After several days of storage the granulated from the upper part of the burner ports within a few hours after material drained to a moisture content of about 5%. operation was started and was replaced by a layer of solidified The damp, granulated product from the storage areaswas prefused phosphate about '/z inch thick that formed on the waterpared for shipment by screening, drying, grinding, and bagging. cooled shell of the burner ports. The water cooling was inadtiThe product was first screened through a l'/d-inch screen to quate t o protect the steel shell, and the steel shell failed freeliminate ungranulated paterial, such as that which froze around quently as a result of burnouts and warping and splitting nf the the furnace taphole and was subsequently rodded into the quench welds. trough. This oversize material constituted about 1%of the furnace product; it was returned to the furnace for remelting. The minus ll/(-inch product WATER was dried in a rotary, oil-fired dryer. SPRAYS In the initial operation of the plant the OIL dried product was ground to'about 85% of -40 mesh in a ball mill equipped with an air OIL classifier. The grinding characteristics of the material were such that a large percentage of very fine material (-140 mesh) was produced. Since the results of greenhouse and field plot tests indicated that granular, sand-size material was about as effective as finely ground material and since granular material is more readily handled in fertilizerdistributing equipment, facilities were inLUMNITE CEME stalled for producing a granular material by W A T ~ RI1 screening ,the dried product through a 10mesh screen and grinding the oversize, which constituted about 15% of the product, in a hammer mill to pass the 10-mesh screen. The ORIGINAL BURNER PORT PRESENT BURNER PORT product was packed in 100-pound bags and shipped in boxcars. Table IV gives typiFigure 5. Details of Original and Present Burner Ports
I
1
1214
INDUSTRIAL AND ENGINEERING
Table V. Effect of Type and Number of Burners and of Oil Rate on Production Rate, Oil Consumption, and Fluorine Content of Fused Tricalcium Phosphate Kind of Burner Ports
No. of Burners Used
Oil Rate, Gal./Hr
Production Ratea, Tons/24 H r Operating Time
Oil Consumption, Gal./Ton of Furnace Product
114 75 142 81 2 168 87 a Gross furnace production r a t e (not aorrected f o r losses). t.ew
'
2 2
Fluorine Content of Product,
37
0.49 42 0.40 47 0.34 subsequent processing
Tests were made v i t h the original burner ports, two, three, or four burners being fired a t total oil rates of 90 to 175 gallons per hour. The data reported in Table V are averages of 12 to 30 days of operation on burdens consisting principally of brown rock and sinter. When the two front burners were used (the two burners adjacent to the taphole), operation was easier and the oil consumption per ton of product was lower than with three or four burners going. However, the maximum oil rate obtainable with two burners of the original design +as about 117 gallons per hour, and a t this oil rate the fluorine content of the product was often higher than the desired maximum of 0.4%. Increasing the oil rate by increasing the number of burners operated decreased the fluorine content of the product but did not significantly increase the production rate. Figure 5 shows the design of the burner parts in use a t the time this article was written. The new parte are shorter and have a smaller inside diameter (30 and 10 inches) than the original ports (50 and 21 inches, respectively). I n the new ports spiral cooling coils are embedded in the refractory lining. The lining consisted of Lumnite cement mixed with firebrick grog or with granulated fused tricalcium phosphate; both mixtures proved satisfactory since the refractory was protected from melting by the cooling coil. Coincident with the burner port changes, the maximum oilburning capacity of the burners was increased from 60 to 90 gallons per hour by installing larger oil spray nozzles, so that the desired total oil-burning rate could be obtained when only two burners were used. The change in burner-port design resulted in a definite improvement in furnace-operating results (Table V). The production rate was increased from an average of about 50 tons per 24 hours of operating time with the original burner ports to about 80 tons with t,he improved burner ports, and the oil consumption per ton of product was decreased from about 70 to 42 gallons. The fluorine content of the product decreased as the oil rate was increased, bot,h n-ith the improved burner ports and with the original ports, and was consistently 0.4% or less when the total oil rate was higher than 140 gallons per hour. Tapping of the melt from the furnace w s much easier with the new burner ports. The improvements from the use of the new burner ports are attributed to a decrease in the water-cooled area adjacent to the flame; it is believed that t,his decreased cooling area resulted in a higher &ime temperature in the hearth of the furnace. The temperature of the molten product tapped from the furnace was about 50" F. higher after installation of the new burner ports. Moreover, the smallet burner ports appeared to make the entire furnace hearth active, whereas with the larger ports there appeared t o be a n inactive zone in the middle of the hearth; it is believed t h a t the higher velocity obtained with the smaller ports resulted i n penetration of the flame to the middle of the hearth. The velocity of the air-oil mixture in the smaller ports was about 60 feet per second (calculated a t normal temperature and pressure), which ier many times higher than maximum flame propagation
CHEMISTRY
Vol. 38, No. 12
velocities for hydrocarbon-air mixtures for which data have been published. When these burner ports were designed, there wss some doub't as to whether combustion would occur at such high velocity; however, combustion w&s found to occur within the ports. It is not necessary and probably not desirable that combustion be complete in the burner ports, but it is desirable that combustion should start in the burner ports. The use of the internally cooled burner ports afforded adequate protection to the adjacent portions of the steel shell against burning out and decreased the frequency of furnace shutdowns.. The bosh and upper part of the hearth have also been equipped with water-cooling coils embedded in the refractory lining adjacent to the shell. These coils n-ere effective in preventing steel shell failures in these parts of the furnace. Steel pipe coils in the bosh and hearth were corroded rapidly by sulfur gases that were formed by combustion of the fuel oil; stainless steel pipe coils are now being used and have been satisfactory thus far. (Tests have indicated that copper pipe would also be satisfactory. Steel pipe coils were satisfactory for use in the burner ports. The unlined, water-cooled furnace hood originally provided was unsatisfactory because of severe corrosion that presumably was caused by condensation on the interior surface of the steel. This condition was corrected by lining the hood with refractory and discontinuing the use of cooling water on the steel shell. An appreciable amount of silica separated in the furnace when the weight ratio of Si02 to FezOa plus Alaos exceeded 3. For instance, silica separation occurred when the product contained 26% Si02 and 8% Fez08plus ALOZ. The silica separated in the form of plastic lumps, which were tapped out with the molten product. When the melt contained a large proportion of the siliceous lumps, tapping was difficult because the lumps frequently lodged in the taphole and had to be rodded out. The siliceous lumps were incompletely disintegrated in the granulating system and occurred in the quenched product as white, rounded lumps ranging from up to 3 inches in diameter. The proportion of silica in the charge had no obvious effect on the completeness of defluorination; however, the silica content in all cases was within the range that laboratory-scale experiments ( 8 ) indicated to be favorable for rapid defluorination. BRIQUETTED OR EXTRUDED CHARGE
Most of the Tennessee phosphate is recovered in the form of washed sand which must be agglomerated to provide a suitable furnace charge. Phosphate sand that h.as been agglomerated by sintering is a satisfactory furnace charge; however, it is believed t,hat an equally satisfactory and less expensive charge can be prepared by wet agglomeration. In the method used in preparing charge for the pilot-plant furnace, phosphate sand containing about lS70 moisture was rendered plastic by grinding in an edge runner for about 15 minutes, and then the moist material was extruded in a standard extrusion machine equipped with a desiring chamber. In the full-scale plant, extrusion machines were provided that 'were similar to those used in the pilot plant, but a type of grinder was installed which did not impart the necessary plasticity to the sand to permit extrusion. hbout 200 tons of extruded pellets were made by make-shift methods and charged to the furnace; furnace operation was satisfactory with this extruded pellet charge. Briquetting of phosphate sand has been studied, using an experimental roll-type briquet press of about 5 tons per hour capacity. \Then the damp sand was ground a few minutes in a wet pan, it could be briquetted without binder and the briquets appeared to have adequate strength for use as furnace chyrge. The feasibility of using briquetted raw phosphate as furnace charge is indicated by the Victor Chemical Company's blast-furnace operations (5) in which a similarly briquetted charge was used for several years with satisfactory results. It is planned to make further tests of briquetted and extruded charges in the furnace.
December, 1946
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
5
1215
PHYSICAL A N D C H E M I C A L PROPERTIES
Fused tricalcium phosphate in the granular form resembles river sand in appearance except for its color which is usually grayish green. After several months of storage in the open there was no detectable change in physical properties or chemical composition of the product. It does not cake, is not hygroscopic, and is not water-soluble. The dried, -10-mesh product has a bulk density (loosely packed) of about 75 pounds per cubic foot. Tests by a procedure described in a previous paper (19) showed that both granular and finely ground fused tricalcium phosphate had satisfactory drillability; the drilling rate was about the same a~ for Chilean sodium nitrate (Champion brand) and was less affected by exposure to humid atmosphere. Typical analyses of the fused tricalcium phosphate produced at the Columbia plant follow (in per cent) : Si01
26.9 28.5 27.7 29.6
CaO 38.6 40.2 39.9 42.0
Fen08
AlrOi
F
4.2 4.4 4.7 3.0
4.6 4.0 3.9
0.34 0.34
PI01
4.7
25.3 22.2 23.4 20.1
1
0.18 0.38
Examination of fused tricalcium phosphate by microscopic and x-ray methods has shown that the quenched product consists chiefly of ultramicroscopically intergrown a-tricalcium phosphate and a glassy material that contains calcium silicate and oxides of iron and aluminum. The product also contains cristobalite (SiOz) and microscopically visible crystals of fluorapatite in varying quantities governed by the fluorine content. Although fluorapatite cannot be distinguished from hydroxylapatite or fluorhydroxylapatite by x-ray methods, the refractive indices are different and the apatite material found had a refractive index corresponding to that of fluorapatite in the few cases in which the refractive index could be determined. Cristobalite is the principal constituent of the siliceous material that separates from high-silica product. p-Tricalcium phosphate was found in the unquenched product and sometimes was observed in small quantities in the quenched product. Examination of the residue remaining after leaching with 2% citric acid showed t h a t the fluorapatite was not dissolved and that most of the a-tricalcium phosphate w&s dissolved. The percentages of a-tricalcium phosphate that were not dissolved by citric acid were usually small and appeared to vary with the type of glassy impurity with which the tricalcium phosphate was in-
1216
INDUSTRIAL A N D ENGINEERING CHEMISTRY
tergrown. The different types o i glii>sy material weir di.tinguishable by their appearance but were not idcntificd.
Vol. 38, No. 12
of product.
This requirement does not include maintenance, lubor, administration, or operation of the plant service facilities othc'r than the air, steam, water, and fuel-oil systems.
USE AS FERTILIZER O R A N I M A L FEED SUPPLEMENT
Several hundred tons of the pilot-plant product mere used i i i field plot test's that included a wide variety of soil types and moyt of the important farm crops. These tests, conducted by the agricultural experiment stations o f several sout,heastern states, shoived that fused tricalcium phosphate gave yield increases approximating those obtained with superphosphate, per unit of total PlOj. There is no accredited analytical procedure for evaluation of PzOj aT7ailability in fused tricalcium phosphate. About SO?; of the P z O content ~ of fused tricalcium phosphate (0.4% fluorine) is soluble in citric acid by the Kagner method when modified by fine grinding of the sample; about 7jC; is soluble in neutral ammonium citrate. Seither method gives as favorable comparison with superphosphate as the actual crop responses obtained in t,he field plot tests. The product from the Columbia plant is distributed on the basis of total P z O content, ~ with the specification that the fluorine content shall not exceed 0.4% (11); it is believed that this method is a more accurate criterion of its effectiveness than any chemical method thus far developed. Kat,her extensive tests have been made of the value of fuietl tricalcium phosphate for animal feed supplement with rats ( 2 , 6, 9, I O ) , chickens, pigs, and cows; t'hese tests have indicated that fused tricalcium phosphate is effectjive in supplving phosphorus and calcium for animal growth. Most of the fused tricalcium phosphate used for f eding testa coqtained 0.1% fluorine or less. The Association of American Feed Control Officials ( 1 ) suggests that' defluorinated phoaphates (calcined, fused, or precipitat,ed calcium phosphates) intended for animal feed supplement should contain not more than 1 part of fluorine to 40 parts of phosphorus; this is equivalent to a fluorine content of 0,3y0or less for fused t'ricalcium phosphate containing about PpOs. However, this is an emergency specification and may not continue in effect. There is considerable evidence (6, 9, 17) that fused tricalcium phosphate containing more than O.lyofluorine but less than that specified above m a y be used for animal feeding without deleterious result's. PROCESS REQUIREMENTS
The Eollo~inginformation, based 011 operatioli of the Columbia plant,, is intended t o facilitate calculat'ion of production costs for various locations. Figure 6 shons layout. of the Columbia plant. About 1.12 tons of furnace charge are required per ton of product when a calcined charge such as sinter is used. If &.raw charge (brown rock or briquetted sands) ii: ujed, about 1.17 tons of charge would be required because of the litrger percentage of volatile ingredients in t,he charge. These quautities allow for furnace losses and minor losses in subsequent, processing. Since no P105 is volatilized in the furnace, the Pz06 content of the product is somewhat higher than that of the charge. The product should contain 26 to 30% P205 (depending 011 the grade of raw material), 38 to 42% CaO, 20 to 25y0 SiOl, and 7 t o 12% FezOs plus A1203. About 50 gallons of fuel oil and 70,000 cubic feet of air (Sormal temperature and pressure) are required a t the furnace per ton of product. (If natural ga,s were used as fuel, the quantity required presumably would be that needed t o supply an equivalent quantity of heat.) The oil requirement for product drying was 5 gallons per ton of product, and about 3 gallons per ton of product were used to generate steam for heating the oil and the buildings. The electric power requirement was 130 kw.-hr. per ton of product, and the water requirement (for furnace cooling and gmnulation) was 4000 gallons per ton of product. The direct labor requirement for operating the Columbia plant a t full capacity (150 tons per day) ic. about 2.7 man-hours per ton
CONCLUSIONS
The phosphorus content of rock phosphate can be made available for plant nutrition by fusion and defluorination in an oilfired shaft furnace. The furnace product can be granulated when tapped from the furnace to sand-size material, of which only a small proportion need be ground for fertilizer use. The production of fertilizer by fusion and defluorination of rock phosphate has several advantages over other processes. Since the product is not water-soluble or hygroscopic, it may be stored out of doors and shipped in open cars and may be bagged in inexpensive paper bags. The product is more concentrated than ordinary superphosphate, does not require acid or highgrade phosphate rock, and probably can be produced a t a lower ('ont in many localities, particularly where fuel oil or natural gas arid phosphate rock are available a t low costs. ACKNOWLEDGMENT
The development of the process described in this paper was rarried out by the TVA Department of Chemical Engineering; members of this department assisting in the development were W. H. MacIntire who conducted greenhouse tests, J. N. Junkins I+ho helped in the design of the Columbia plant, H. R. Mosley and 5'. S. Wildsmith who are in charge of operation of the Columbia plant, and H. J. Kerr, M. R. Siegel, and many others who assisted in pilot-plant development work and in preliminary operation of the Columbia plant. The Department of Chemical Engineering was assisted by the Design Department in the design of the Columbia plant and by the Department of .4gricultural Relations and agricultural experiment stations of the states in the Tennessee Valley area and elsewhere in agroiiomir and animal feeding tests. LITERATURE C I T E D
(1) hssoc. of Am. Feed Control Officials, In?.. Ofiicial Puhlication, 1946. (2) Bird, H. R., Mattingly, J. P., Titus, H. W., Haminond, J. C., Kellogg, W. L., Clark, T. B., Weakley, C. E., Jr., and Van, A. H., J. Assoc. O$lciaZ Agr. Chem., 28, 118-29 (1945). (3) Curtis, H. A . , Copson, R. L., Brown, E. H., and Pole, G. R., IXD. ENG.CHEM.,29, 766-70 (1937). (4) Curtis, H. A, Miller, A. M., and Newton, R . H., Chem. & Met. Eng., 45, 116-20 (1938). (5) Easterwood, H. W., Ibid., 40, 283-7 (1933). (6) Ellis, N. R., Cabell, C. A, Elmslie, W.P., Fraps, G. S., Phillips, P. H., and Williams, D. E., J . Assoc. Oficinl Agr. Chem., 28, 129-42 (1945). (7) Elmore, K. L., U. S.Patent 2,368,649 (1945). (8) Elmore, K. L., Huffman, E. O., and Wolf, W. W., IND.ENO. CHEM.,34, 40-8 (1942). (9) Fraser, H. F., Hoppe, T. C., Sullivan, J. H., and Smith, E. R., Ibid., 35, 1087-90 (1943).
(10) €Till, We L., Reynolds, D. S.,Hendricks, S. B., and Jacob, K D., J . Assoc. Oficial Agr. Chem., 28, 105-18 (1945). (11) MacIntire, W. H., Winterberg, S. H., Hatcher, B. W., and Palmer, G . , Soil Sci., 57, 423-42 (1944). (12) Marshall, H. L., Reynolds, D. S.,Jacob, K. D., and Rader, L. F., IND.ENO.CHEM., 27, 205-9 (1935). (13) Miller, P., Lenaeus, G. A., Saeman, W. C., and Dokken, M. N., Ibid., 38, 709-18 (1946). (14) Reynolds, D. S., Jacob, K. D., and Rader, L. F.. Ibid., 26,406-12 (1934). (15) Ibid., 27, 87-91 (1935). (16) Reynolds, D. S., Marshall, H. L., Jacob, K. U., and Rader, L. F.,Ibid., 28, 678-82 (1936). (17) Williams, D. E., McLeod, F. L., Morrell, E., and Jones, F. P., I b i d . , 38, 651-4 (1946). PRESXIXTLD before t h e Division of Fertilizer Chemistry a t t h e 110th 'Cleeting of the . ~ > I E R I C A KCHEMICAL S O C I E T Y , Chicago, 111.
