STEEL WET STORAGE TRESTLE WITH DRYMILL AND
. l I ~ z l b e r r ~Cia. ~,
STORAGE
BIN IN BACKGROUND
The PHOSPHATE ROCK INDUSTRY of the UNITED STATES. I1 WILLIAM H. WAGGAMAN 801 Hollingsworth Avenue, Lakeland, Florida
Part I of this @per discussed the chemical nature, probable geological origin, and economic signij5cance of phosphate minerals and described briefly the princi@l American deposits. Part 11 outlines the chief methods of mining phosphate rock and preparing i t for market. Recent methods of recovering and processing phosphate minerals and the uses of products obtained are discussed.
. . . . . . MWING AND
OF MARKET
T
HE methods employed in mining and preparing phosphate rock for the market depend on the character of the material and its mode of occurrence. There is a great similarity in the methods of handling Florida hard rock, Florida pebble, and certain deposits of disintegrated brown phosphate in Tennessee, but the various steps diier as to details. These three types of phosphate occur either in flat or gently rolling country and are usually covered with an overburden of such a character as to permit its removal by steam shovel, drag-line, or hydraulic means. Where the drag-line can be used i t is preferred, as this machine makes i t possible to remove enormous tonnages a t a very low cost. This immense piece of equipment moves slowly along the phosphate deposit, stripping a section fully one hundred feet in width and dumping the overburden onto a spoil bank usually located in a parallel mined-out pit. No cars are required to haul the top soil away as when a steam shovel is employed. A .
After the overburden is stripped, the method of miuing the phosphate matrix depends on the nature of the rock exposed. In the case of Florida hard rock, the matrix is either dug by steam shovel or dredge, depending on whether the deposit is above or below the normal water table. With this type of rock, hydraulic mining is impractical because of the large bowlders of ohosohate (some of them weiehine several tons) which occur in the deposits. These must often be blasted before they can be loaded into cars. The matrix is loaded into tram cars, hauled up a steep inclined trestle to the washer plant, and dumped upon a grizzly, or eratinz. ". which allows material two inches or less in diameter to pass through into the washer. The larger pieces are broken up with sledges and are also passed through the grating by being. sprayed with heavy streams of water. In the case of the Florida pebble phosphate as well as certain deposits in Tennessee, hydraulic mining is employed because the size of the phosphate particles is such that they can be readily pumped through a ten- to twelve-inch pipe line. In hydraulic mining heavy streams of water under a pressure of one hundred to one hundred sixty pounds per square inch wash the banks of matrix down into a sump where it is taken up by centrifugal pumps and conveyed through heavy steel pipe lines to the washer plant. In deep mines, or where the washer is a long distance from the mine, a second pump or "booster" is often required to raise the material to the proper height.
. .
476
The washer plants also vary in detail but the most modern are of all-steel construction. More elaborate washers are employed as a rule in the pebble fields than in the hard-rock area. Space forbids full descriptions of the various types of washers, but the essential features consist of aseries of vibra.ting and rotating screens followed by several pairs of logs thirty feet or more in length, which rotate in opposite directions in an inclined trough. A series of blades or teeth bolted to the logs in the form of a spiral knead or work up the matrix as i t enters the lower end of the trough and carry it forward while streams of water are continually sprayed upon the mass to remove the adhering clay, sand, and soft phosphate. The water carrying the fine foreign material in suspension flows back to the lower end of the trough and is discharged through a flume onto a waste heap. From the logs, the partially cleansed phosphate passes through revolving cylindrical rinsers of perforated metal where i t is sprayed with further quantities of water and the remaining particles of adhering impurities removed. A separation of coarse and fine particles of phosphate is made in these rinsers. Dorr classifiers or rakes are used in some of the plants for effecting a separation of the granular material from the slimes, and separating tables which depend for their effectiveness on slight differences in the specific gravity of the fine phosphate and silica sand are also employed. The Florida hard-rock plants usually run their coarse material over a "pickling belt" where skilled laborers remove flint rock, clay balls, and limestone from the phosphate, but such a scheme is impracticable in the pebble phosphate plants because of the small size of the product. The flow-sheet of a typical phosphate washer in the pebble fields is shown in Figure 1.
Of the total material handled from a deposit of hard rock phosphate, after the overburden is removed, an average of not more than 20% is recovered as marketable rock. The rest of the material, consisting of sand, clay, soft phosphate, and finely divided rock is discharged into waste ponds. The actual quantity of PzO, thus discarded is fully as great as that saved and marketed as high-grade rock. In the pebble fields, before the introduction of the "flotation process" for separating very fine particles from the silica sand, the average recovery of marketable rock was in the neighborhood of 20%, although individual deposits gave a higher yield. Flotation has also been applied to certain deposits of brown rock in Tennessee and a product of excellent grade thus recovered from material formerly considered of little economic value. The general procedure employed in flotation is described further in this paper. Tennessee blue rock, the Arkansas phosphates, and the deposits in the western states are usually mined or quarried like seams of coal or limestone. Where the beds are more or less horizontal, main tunnels are driven into the phosphate and at regular intervals, rooms are turned off a t right angles, the overlying strata being supported either by heavy timbers or by leaving pillars of rock. The rock is mined by drilling and blasting, and the slabs or lumps are subsequently broken up by sledges, loaded into tram cars and hauled to the surface. Where the phosphate occurs as sharply inclined strata, as is the case in many of the deposits of western phosphate, stoping is resorted to, the phosphate flowing by gravity into main tunnels where it is loaded directly into cars.
Pit Pump 4 c Punched Screen. Stationary I Plls" Opening)
J.
4
Undersize
Oversize
1
+Oversize
Coarse Trammel (1" Holes)
I
1
Undersize
-0vedow
I
I
-Undersize
1
c Log Washex
-
4 1
Elevator Vibrating Screen Ton Cap 35' Pitch)
*I
\ m '
Debris Sumo
4 -4 Sand Sump 4
Oversize Elevators
Hydraulic Classifiers
1
Waste.
1
A
I
Wet Bin
FIGURE1.-FLOW-SHBET 01. A TYPICAL PHOSPHATE WASHERIN THE FLORIDA PEBBLE FIELDS
A PILE OF TENNESSEE BROWNROCKPHOSPHATE Note the wide variation in the size of the material.
shipment. Special hopper-bottom cars with closed tops are employed in the pebble fields of Florida for transporting the dried rock to the ports where modern elevators load i t into vessels for shipment abroad and to the fertilizer plants in this country.
.. ......~. .... ,. ,.
A DREDGE MINn
I N THE
- ,. . .
FLOR~DA HARD-BOCK lill;~~S
Bedded phosphate deposits which are not very porous seldom require any washing and very little fuel is required to dry them, hut underground mining is more costly than the stripping of a medium thickness PROSPECTINGFOR HARD-ROCK PHOSPHATE I N FLORIDA of loose overburden and the removal of the phosphate by hydraulic means. Practically all phosphate rock containing appreciable RECENT ADVANCES I N THE MINING AND TREATMENT OF PHOSPHATE ROCK percentages of moisture is dried by passing it through During the past ten to twelve years distinct advances direct-fired rotary kilns heated by oil or coal. These kilns are similar to those used in the cement indus- have been made in methods of recovering phosphate try, hut, as a rule, are much shorter (twenty-five to rock a t the mines, and in processes for treating the thirty feet in length) and have no refractory lining. mineral to produce phosphoric acid, phosphate salts. They are equipped with flights which shower the rock and phosphatic fertilizers. Some of these improvethrough the flame and hot gases, thus facilitating the ments have not only lowered the cost of mining the drying operation. Such kilns have a capacity of from crude material but have made i t possible to recover or twelve to fifty tons of dried rock per hour, depending use phosphate rock which was formerly wasted, and on their size, the character of the rock, and the percent- thus their introduction is equivalent to increasing our age of moisture contained therein. reserves of this essential mineral. The Drag-line.-The electric-driven drag-line for The rock is usually dried down to a moisture content of one per cent. or less and loaded into dry bins to await the removal of overburden has been one of the outstanding improvements in the mining of Florida and Tennessee phosphates. T h i s enormous piece of equipment which weighs fully four hundred tons, contains a 450-horsepower motor-generator set which supplies a variable direct current to the several motors mounted therein, each of which drives a machine performing a differentoperation. The machinery is so well coordinated, however, that an eaperienced operator can fill the heavy steel scoop (of ten tons capacity), hoist the material from fifty to one hundred feet in the air, swing the one hundred sixty-foot 1a.tticed steel boom, and dump the contents of the bucket on the spoil bank in one minute or less. ili"i6crry. ila. The drag-line has been almost DRAG-LINE REMOVINO OVERBURDEN IN SWAMPYGROUND universally adopted in the This drag-line has a 165-foot boom, and uses a 6-cubic yard bucket. Total weight of Florida pebble-phosphate fields, machine is 820.000 pounds.
and has so lowered the cost of removing overburden that deposits formerly considered unworkable are now being profitably exploited. Under good average working conditions, the cost of stripping by this method is considerably less than five cents per cubic yard. Flotation.-The application of the principles of flotation to the Florida pebble phosphate and to certain deposits of brown rock in Tennessee is another outstanding development. Mixtures of very fine phosphate granules and silica sand (from 20 mesh down to 200 mesh) can thus be separated and a highgrade phosphate product recovered which was formerly lost l,z,mzaiim,d ' l r i < d f M d C0"D S I , d h " " ~ .PI,, where only washing and screenHYDRA~LIC MININGOPERATIONS ing were employed. Moreover, The nozzle pressure of these hydraulic guns is 185 pounds per square inch. flotation makes it economically practicable to re-work many of the old "debris dumps" material (+24 mesh) separated and ground in a rod which for many years have been regarded as almost mill to flotation fineness. Some further de-sliming is worthless, and thus has greatly lengthened the life of usually necessary, and then the thoroughly washed the Florida phosphate deposits. This process is thor- granular mixture is elevated to large storage bins. oughly covered by patents.'= This so-called pulp is drawn from storage and passed The flotation process as practiced in the Florida through a cylindrical mixer where i t is coated with the pebble-phosphate regions by the largest producer of flotation reagents, consisting of a mixture of fuel oil, phosphate rock may be briefly described as follows. pine oil, alkali, and fatty acids. The matrix as i t is pumped from the pits is put The treated mixture of fine phosphate pebbles and through the usual washing and screening process but all silica sand is then pumped to the first of a series of deep material passing through the fine screens usually flotation cells containing rapidly revolving beaters employed, as well as the "back wash" from the logs, which agitate and aerate the pulp. A froth is thus is discharged into settling tanks where the bulk of the formed to which the particles of phosphate cling, and as slime is removed by overAowing into a launder, and this froth rises to the surface of the cells, i t is continnhence through a flume to the waste pond. The fine ously skimmed off by mechanically operated paddles. granular material in the settling tanks then passes Practically all of the silica sand drops out in this first through a series of Dorr rake classifiers where it is cell, but it carries down with i t some phosphate parpartly de-watered and further quantities of slime re- ticles. The settled material, therefore, must he treated moved. It is then run over screens and the coarser to recover further quantities of phosphate, so the rejects from the first cell are treated in a second, those IZ U. S. Patents Nos. 1,547,732; 1,761,546; 1,780,022; 1.795,100. from the second in a third, the roughing operation being repeated five times before the phosphate is entirely separated and the residue discharged as a nearly pure white silica sand. The separated phosphate, or concentrate, is then elevated to steel bins, the surplus water drained off, the product loaded into cars and shipped to the drying plant. Not only is the flotation product often of a higher grade than the coarser pebbles obtained from the same deoosit~, bv the ~TRESTLE AND WASHERPLANT IN THE FLORIDA HARD-ROCK FIELDS usual washing and screening
process, but this method of separating the phosphate from the sand makes it practicable to exploit deposits which otherwise could not be economically handled. This is well illustrated by the data given in Table 3, which shows the recovery of phosphate product from two mines in the Florida pebble region. In the fist case (Deposit No. 1) both flotation and the usual washing and screening process were employed, while in the other case (Deposit No. 2) washing and screening only were used in separating the rock from its associated impurities.
FURNACE PROCESSES OF TREATING PHOSPHATE ROCK
The smelting of phosphate rock in electric and fuelfired furnaces, with the direct recovery of phosphorus or phosphoric acid thus produced, has been another distinct advance in the phosphate industry. This general scheme has two outstanding advantages over the older method of producing phosphoric acid by treatment with sulfuric acid. (1)Low-grade phosphate deposits, formerly considered of little commercial value, and "run of mine" phosphates can thus be utilized, and P206 values recovered even from colloidal phosphates to the separation of which flotation is not applicable. (2) TABLE 3 COMPARISON on TRB RECOVERY 0s PAOSPAATB P~BR *=DM L ~TWO DBPOSITS, Phosphoric acid of a rather high degree of purity and ON= on warcn WAS 8smo MINED IN TAB USUAL MANNER, AND THB OTHBE of any desired concentration can be produced without P U T TARDUGA TAB FLOTATION PLANT the necessity of costly evaporation. Deposit NO.1 s D10aril No. 2 (Floiolian (Wilhoul The basic principles are practically the same, whether Employed) Flalolion) the electric furnace or the fuel furnace is employed, each Hours of operation....................... Total Cubic Yards Handled.. ............. method of smelting having certain advantages and disCubic Yards per Hour.. .................. advantages. The choice of the type of furnace depends Tons of Pebble (without Flathfionl......... Tons of Concentrates (Plotation Product) . . on the relative cost of power and fuel, and the quantity Total Tons of Pebble and Coneentratcs... . . ' of acid which it is desired to produce. Cubic Vardn Matrix per Ton of Pebble.. .... Cubic Yards Matrix per Ton of Concentrates The fundamental reactions involved are the practiCubic Yards Matiir oer Ton of Pebble and cally complete reduction (at high temperatures) of cal..... Concentrates .......................... 6.3 Tons of Pebble per Hour.. ................ 29.6 78.2 cium phosphate, in the presence of silica and coke, to Tons of Concentrates per Hour.. .......... 22.0 ..... 61.6 78.2 Total Tons of Product per Hour.. ......... elemental phosphorus, and the production of fusible 72.84 8.e.~. Croda of P ~ b b l cR c c o o ~ r e d . .................. . calcium silicates. In their simplest form, these reacG m d c of Conrrnlrale R c c m r c d . . .............. 75.16 B.F.L. tions may be represented thus: Without Plotation, Deposit No. 1 could hardly be worked economically. 5C 3SiOs = 3CaSi01 5CO Pz (1) Caa(PO4)a Deposit No. 1 could hardly be commercially utilized 0
+ +
without the application of the flotation process, since nearly one-half of the phosphatic material is so fine that it passes through the screens along with the silica sand. The yield of pebble phosphate from the matrix of mine No. 2, however, is well above the average, and such a deposit can be profitably mined without the application of the flotation process. By use of the flotation process, however, the yield of marketable phosphate from this latter deposit could unquestionably be materially increased and at a lower cost per ton.
(2) (3)
+
+
+
2Pa 502 = PXOI = 2H3PO4 P20s 3&0
+
The elemental phosphorus is driven off along with the carbon monoxide, and is either oxidized directly to PzOs, humidified, and converted into phosphoric acid, or the phosphorus is condensed out of the furnace gases and thelatter used for the production of heat and power, the phosphorus being subsequently burned in a separate combustion chamber. In either case, the phosphoric acid is collected in the form of relatively strong acid by a Cottrell electrical precipitator or by absorption towers. The molten calcium silicates produced in the furnace are tapped off from the furnace in the form of a slag. The relative merits of the fuel and electric furnaces and the "one-step" and "twostep" processes for making the phosphoric acid have been discussed in a recent paper by the writ~r.'~ Thrre arc at least thrre large plants in this country primarily designed to produce phosphorus and phosphoric acid by the fumacc or volatilization process: one in .4hbama employing the elcctric furnace, and om each in
c o s r i r"1~iiooirr ~ and MINING TENNESSEE BROWNROCKPHOSPI~ATE
.
.1luron
-.
WAGGAMAN, W. H., Ind. Eng. Chef%.. 24, 983 (1932).
Florida and Tennessee using the fuel furnace. While a small percentage of the phosphorus thus produced is marketed in elemental form, the bulk of the element is converted into phosphoric acid, a product which is annually finding a wider use for industrial purposes. Simple flow-sheets of the two modifications of this process are shown in Figures 2 and 3. Phos. Rock
Silica
r
4
Hot Blast
Blast Coke i---tFurnace t-
I
-
Hot Blast Stoves I
-
(forR. R. Ballast) or Concrete
Blast
means, and stored in large piles to cure. Curing usually requires from two weeks to severd months, and during this time the mass tends to crystallize and usually sets to such an extent that i t must be again broken up and pulverized before it can be shipped. It can be seen,
$ :"',:
J. COa & P%06
Water
-
or Steam
I
J.
Phos. Rock I
Spray Tower
Silica
r
Hot Blast
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Cottrell Precipitator coz I , (Spent Gas) HIPO~ (to Storage) FIGURE2,-FLOW-SHEET or THE ONE-STEPPROCESS OP PRODUCING PHOSPHORIC A a D BY THE BLAST-FURNACE METHOD,THE PHOSPHORUS BEINGOXIDIZEDWITH THE OTHERGASES
4
IMPROVEMENTS
IN
THE MANUFACTURE PHOSPHATE
OF
Superphosphate still continues to be the main phosphate product used in the fertilizer industry, but during the past five years steps have been taken to improve the process of manufacture and the physical and chemical nature of the product. The old den process, however, is still employed in most of the fertilizer factories. Before enumerating these new steps and processes, a brief description of superphosphate manufacture as practiced for so many years may not be amiss. The phosphate rock is fmt finely ground to 60 or 80 mesh, and thoroughly mixed with slightly less than an equal weight of sulfuric acid (50' to 5ZoB6.). The mechanical mixers used for this purpose hold from one to two tons, and are so arranged that after being stirred for two to three minutes, the muddy mass is discharged through a central opening in the bottom of the mixing pan into a closed chamber or den immediately heneath. Here the reactions, which were begun in the mixing pan, continue and large volumes of gas and water vapor are evolved. The mass is allowed to remain in the dens for aooroximatelv twentv-four hours. and is then dug out either by hand or b i mechanical
.-
I
SUPER-
J.
Air or ------------t
Oxygen
IZii T"
P. cornbust. Chamber
I
J. Pl00 I
Water or Steam
J.
P. Storage CO Gas (for Power)
Hot Blast
*
cold
i
Blowing
cEngine Blast
coz
(Spent Gas)
$ , Humidifying Chamber I
J. Cottrell
Precipitator I
J.
Hap01 (to Storage) FIGURE 3.-FLOW-SHEET OB THE TWO-STEP PROCESS O P PRODUC I N G PHOSPHORIC ACID BY THE BLAST-FURNACE METHOD,THE PHOSPHOROUS BEINGCONDENSED AND OXIDIZED IN A SEPARATE COMBUSTION CHAMBER
The product is discharged from the autoclave in the form of small, dry, porous balls, which can he readily crushed, yielding a granular superphosphate with much less tendency to set than that prepared by the den process. The material runs uniformly through the fertilizer drills, thus insuring proper distribution in the field. Since this process requires a smaller proportion of acid than that ordinarily employed, and the product contains less water than that produced by the den process, it carries a higher percentage of P206than a den product manufactured from the same grade of phosphate rock. It is also much denser and thus requires less storage space and smaller bags for a given weight. One plant in Baltimore is equipped with a battery of three autoclaves for turning out this type of superphosphate, and two plants in Canada have adopted the same process. Ammoniated Superphosphate.-Still another improvement in the manufacture of phosphatic fertilizer, which has been widely adopted by the industry, consists in the treatment of superphosphate, or mixed fertilizers containing superphosphate, with either ammonia gas or ammonia liquor.14 After the superphosphate has been cured, it is either sprayed with ammonia liquor, or exposed to the action of a definite quantity of ammonia gas until i t has adsorbed sufficient to neutralize the greater part of the free acidity and to convert a portion of the P,Os present into ammonium phosphate, ammonium sulfate, and dicalcium phosphate. Care must he taken that too much ammonia is not added, as insoluble phosphates will result, approximating the composition of the original rock. The maximum quantity of ammonia which "JACOB, K. D. AND ROSS,W. H., J. A m . Soc. Agron., 23, 771-87 (1931): KEENAN.F. G., Ind. Eng. C h . , 24, 44-9 (1932); KITSUTA,K. AND SALTER, R. M., ibid., Anal. Ed.. 3, 3 3 1 3 (1931); KR~~GEL, C., Ammonieted Super~hosphate,4 , 261-6 (1931); PARKER, F. W., Commerdal Fertilizers, 42, 2-4
therefore, that where a large business is anticipated, immense storage facilities are required to have an ample supply of cured superphosphate ready for shipment when the fertilizer season opens. The usual grade of this superphosphate material contains 16% of available P20s,which means that i t must be soluble either in water or in a neutral solution of ammonium citrate. Superphosphate is either used "straight" or mixed with carriers of potash and nitrogen to make up complete fertilizers. The Autoclanre or "Oberphos" Process.-A new process for the manufacture of superphosphate has been developed by a firm in Baltimore (G. Ober & Sons Co.) whereby an economy in the quantity of sulfuric acid normally employed is effected, and a finished product, cured and ready for the market, is obtained within twenty-four to forty-eight hours. This process may he briefly described as follows. The sulfuric acid and finely ground phosphate rock (in the proportions of eighty to eighty-four parts of acid to one hundred parts of rock) are simultaneously forced (by air pressure) through a single orifice into an evacuated, rotating, steam-heated autoclave. The autoclave is then sealed, and the gases and steam evolved from the reactions allowed to build up pressure within (1931). the container. The time of mixing of the acid and rock may thus be much prolonged over that possible in the open mixers, where the loss of water through evaporation soon causes the material to "set." Moreover, the chemical heat is not dissipated and the reactions can proceed a t a higher temperature without loss of water, thus insuring more complete decomposition of the phosphate rock by the acid. When the mixing has continued for a sufficiently long time to insure the maximum reaction between the ingredients of the slurry, the pressure is L released, and the material dried by the application of vacuum. PHOSFTIATE FLOTATION PLANT
Phoaphalc Recovny carp., M u l b n r y , Flo.
can be added to the average superphosphate, without causing reversion, is in the neighborhood of 3.5%. This process not only gives a very uniform distribution of ammonia in the product, but utilizes the acidity of superphosphate to fix a certain proportion of ammonia which otherwise must be added to mixed goods in the form of ammonium sulfate or other nitrogenous
ess or by furnace treatment of phosphate rock, has a wide variety of uses and is a constituent of many industrial products. These products and their uses are listed in Table 4 in the order of their present comTABLE 4 P m s ~ m m cAcro nwo ITS P e o o u n s ( O r a m T n m Srr~sasnospnars) LISTEDIN TEE ORDBRon T ~ I PRESENT R C O U M ~ R CI U ~A PL YRTANC~
STRUCTURAL STEELWASHERAND WET DINS
product. The lack of free acidity in such a product and its low content of free moisture also prevents the material from setting and permits it to be uniformly distributed in the field. The latest suggested modification of this scheme, where a higher nitrogen content is desired in the 6nished product, consists in adding to the snperphosphate a solution of urea in ammonia liquor.'Vn this way the nitrogen content of the product may be much enhanced, a uniform distribution of this fertilizer ingredient effected, and the resultant product will have the advantage of containing both organic and inorganic nitrogen. VARIOUS U S E S OF PHOSPHORIC ACID AND PRODUCTS OBTAINED THEREPROM
Phosphoric acid, whether produced by the wet proc-
l6 PARKER, F. W. 39, 540-1 (1932).
AND KEENAN, K .
G.,Cl~em.& Met. Eng.,
PIdYCl
Triple Superphosphate Monocalcium Phosphate Trisodium Pho.phate Ammonium Phosphates Phosphoric Acid Diaodium Phosphate Sodium Acid Pyraphosphate Dicalcium Phosphate Tricaleium Phorphate Zinc Phosphate Silver Phosphate Tricresyi Phosphate
uses Fertilizers Baking powder water softener, washing powders Fertilizers, yeast culture. baking powder. fire-prmfing Rurt-proofing, beverages, medicinal Medicimd, weighting of silk, yeast culture Baking powder Tooth p a t e , mild abrasive T w t h paste, mild abrasive, salt conditioner ~enta
