Modern Plant for Treating Florida Phosphate Matrix Large settling area provided for slime disposal is a mile or more from this plant
Phosphatic Slime A POTENTIAL MINERAL ASSET PAUL M. TYLER AND WM.H. WAGGAMAN Minerals and Metals Advisory Board, National Research Council, .VationaZ Academy of Sciences, Washington, D. C .
T
HE phosphate rock industry is an outstanding example of how the application of modern mining and ore-dressing methods has increased the output and recovery of a vital mineral product. This is particularly true of the Florida pebble deposits, where up-to-date equipment and efficient processing techniques have made it possible to expand production to keep pace with the phenomenal increase in demand for one of the three most essential plant nutrients. These new developments have nearly tripled the life of this phosphate field by making it commercially practicable to mine deposits that formerly were considered entirely out of the economic picture. Yet in spite of the advance in the price of labor and equipment, the over-all production cost of this mineral per ton has not shown a proportional increase. The annual production of Florida phosphate has increased in the past 20 years from 1,500,891 long tons in 1932 to 8,211,820 long tons in 1951. Although more than 82,000,000 tons of commercial rock have been produced and sold during this period, measured reserves based on present methods of recovery are enough for many decades a t our present rate of consumption. Sotwithstanding major advances in mass production of this relatively low priced commodity, opportunity for further technologic improvements is indicated by the fact that one third of the phosphorus pentoxide content of the material sent to the washer
is lost in the effluent slimes and additional finely divided phosphatic material is regularly discarded at the mines along with the barren overburden, which has to be removed before actual mining can begin. The economic and technical difficulties surrounding the recovery of this final increment of phosphate are many and complicated; else the industry would have solved them before now. However, the need for a satisfactory disposal of these waste products becomes more pressing every year. I n 1952, the Minerals and Metals Advisory Board of the Xational Academy of Sciences-National Research Council was requested by the Defense Materials Production Administration and the Bureau of hlines to gather all available information that might be helpful in a study of the phosphatic slimes now being rejected at plants that prepare high-grade phosphate rock for the market. The resulting data were submitted in the form of a staff study to a panel of outstanding technologists and engineers set up to consider the problem of slime disposal and utilization. M. D. Hassialis, Department of Mineral Dressing, School of Mines, Columbia University, New York, N. Y., presided as chairman. The members of this panel, together with liaison representatives of interested government agencies, met in Lakeland, Fla., March
1049
loso
INDUSTRIAL AND ENGINEERING CHEMISTRY
16, 1953, to discuss this problem and to consider proposals for its solution. The present article summatizes the highlights of the original staff study and the salient points brought out in the discussion at the panel meeting. RECOVERY OF FLORIDA PEUELE PIIOSI'II.I'I'I.:
Prior to 1928, the methods employed in mining and trcating raw phosphate (known as matrix), though simple, were extremcly wasteful. Yet they were generally believed t o be highly eficient. Owing to the lorn value of the product, substantial expendit,ures for improving recoveries were deemed economically inipractirable.
q 324% B P L
Waste
Tailin s 50'.B%L
Vol. 46, No. 5
ivhich passes through the conventional screens, could be desliined and treated with flotation agents, so that the phosphatic particles could be economically separated from t,he quartz sand. This tievelopinent not only more than doubled t,he yield from many deposits, but brought into production deposits that contained I small percentages of coarse phosphate pebble. As greater th nesses of overburden could be removed profitably, larger drag lines \$-ereemployed and the matrix v a s handled by drag line instead of hydraulically. Various types of classifiers were installed after the screens t o remove finer particles, and huge hydroseparators and thickeners were added, to assure that, only t,he slime frac3tion ( - 150 or - 200 mesh) was discharged t o waste without, being treated to recover the phosphate. \There debris from operations had been segregat,ed, it, hecxnie economically practicable to rework old dumps and to recover large tonnages of the fincr sizes of high-grade phosphate rock. Whereas the old system of treatment of phosphate matrix yielded only coarse pebble and debris or raste, the preeeiil P J tcm results in four products-high-grade coarse pebble, flotation concwitrates, relatively barren flotation tailings, and phospliatic slime diluted wit,h great quantities of water. Although at Iwst t K O major process steps have been added, the additional r(zc:o\wy of phosphate values more than off-sets the greater cupital investment, in equipment and the increased operating costs. The life of the Florida pebble field has been greatly extentled 11)- the higher recovery of phosphate in the matrix. Severtheless, an enormous tonnage of potentially valuable material i discarded in the subsieve sizes below about 150 mesh.
~
PHOSPHATE BALANCE Florida Pebble Phosphate Matrix
Figure 1.
Products Derived from Matriv
Proportions of T o t d l BPL in Ptoducts
Florida Pebble Phosphate Jlatrix and Deriyed Products
In thc mining and procesping, the barren ovcrhrtirii. ( a i sieting chiefly of sand, \vas first removed by s c r a p e ~ ' ~strain . shovels, or small drag lines. The underlying phosphai c matrix was then mined wit,h hydraulic jets (monitors), n-arhed into a, sump, and pumped through a pipeline to the washer plant. SIaterial coarser than 20 to 30 mesh was separated by n-ct ~ci.criiing, dried, and finally marketed as high-grade rock. The undersize (under about 20 mesh), 1cnon.n as washrr debris, consisted of a mixture of sand, fine phosphate, and clay minerals, but was discharged without further treatment through a flume into mined-out pits and large artificial ponds v-ith earthen dams, where the suspended material n-as allowed t,o settle, so that clear water could be circulated back to the washer plant for rc-use. Only about 157, of the total matrix handled was recovered as marlretable rock, as no means had been developed for wparatiug the finer sizes of phosphate from the other minerals nit11 which it Tyas associated. Man>-deposits were considered of n o economic value because of thc small percentage of coarse pehblee or the excessive ratio of overburden t o recovered phosphalc. Had there been no improvement in the mining and r c c o ~ pebble phosphate during t,he past 26 >-ears, most of the knowi deposits probably ~ o u l dnow be considered on the vcrge of exhaustion and operators would be looking for new sources of rock t o meet esaent,ial agricultu~,alnnd industrial needs. FLOTATIOS PROCESS
hhout 1027, however, the Florida pebble phosphate industry began to experience a profound change. The introduction of t h e flotation process and its subsequent adoption by all operators not only madc it necessary to revamp the phosphat,e recovery plants but also wrought a revolution in mining techniques. IC was found that the fine granular material (-20 +200 mesh),
hvailable inforniation on the distribution of the pliosplio~~us prii~oxideor its 13PL equivalent in the fractions of Florida prhblc matrix is summarized in Tahlc I, arid shown in diagrammatic form in Figure 1. [In the fertilizer industry, 1 ' 2 0 6 is termed phosphoric acid and RPL is the usual symbol for "bone phosphate," which is the thcoretical caontent of tricalcium phosphate, Ca3(1'0~):..1 The data represent the r e d i s of an investigation condurted in 1048 (6) in cooperation with the seven phosphate operators in the Florida pehhle fields. This test covered 20 days, during n-hich time 1,280,000 tons of mairix wrrc mined and recorda kept of the quantity and phosphate rontent of both recovered and rejected produc?3.
T.IRLEI. FLOHJI).~ P H O S P I ~B.LIAXCE ~TE SHEET (Propoi tioni of recovered z.nd waste products)
BPI- in Matrix", G~~~~ BPI, Content Recovered Products, and Weight, 0; of \Taste. % Tons I on8 total I'hosphate matrix (36% BPL) 100.00 36 0 100 0 Coarse pebble ( 7 2 . 0 % BPI,) 1.83 1.3 3 7 3Iedium pebble ( 7 2 . 0 % B P L j 3.30 2.4 6 6 2 Fine pebble ( ? 2 , 2 % BPL) 8 C O 32 . 7 .~~~. __ Total pebble ( 7 2 . 2 % B P L j 875 0 3 175 C'oarse flotation ( 7 5 . 0 % BPI.) .5.90 4.4 12 2 35.00 11.4 31--i I'ine flotation ( 7 6 . 0 % B P L ) __ Total flotation ( 7 5 , 8 % BPI,) 20.90 15.8 43 9 \Taste tailingsb ( 9 . 0 % BPL) 86.63 3.2 0.0 Slimes ( f 2 0 0 mesh) ( 2 7 . 4 % BPL! 11.00 3.0 8.4 22.70 7 0 21 9 Slimes (-200 mesh) (34.85% 13PL) ~.__ 33.70 10.9 30.3 Total slimes (32.4yo BPL) __ __ Grand total 100 00 36.2 100.7 (1 Bone phosphate of lime or trical(4u:n phosphate, Cas(POaj2. b Tailings from flotation treatment.
,.
~
The ultrafine material or slimes currently discharged as waste rontain approximately one third of the phosphate present in tho original matrix. Tonnagewiee, this represents an annual loss of large proport,ions. There were 8,320,033 long tons of Fiorida phosphate pebble sold or used in 1951, containing the equivalent of approximately 6,250,000 tons of RPL. On the reasonable aseumption that this tonnage represented 61 70of the BPT, in t>he
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1954
original matrix (10,246,000 tons), over 3,996,000 tons of BPL were discarded, 920,000 tons in the form of flotation tailings and 3,076,000tons in the phosphatic slime. Approximately 1.14 tons of slime (dry basis) are rejected for each ton of high-grade pebble phosphate recovered; therefore, the quantity of slime discharged to waste in 1951 was of the order of 9,500,000 long tons. Inasmuch as this slime was pumped away from the washers and flotation plants as a dilute slurry containing only 2% solids, the materials actually handled (solids plus water) amounted to about 450,000,0001ong tons. Thousands of acres of settling area have to be provided, and new slime ponds created from time to time after older ones have been filled and abandoned. These desolate areas continue to be an eyesore and a potential source of stream pollution and damage to adj oining properties. The accumulation of slimes since flotation was introduced has Reen conservatively estimated ut 125,000,000 tons. There is an equally large tonnage of washer debris comprising a mixture of slimes and coarser material (-30 mesh), accumulated before the adoption of flotation. Some of these old dumps are so badly mixed rvith discarded overburden that probably they can never be profitably reworked. Rough estimates of the quantity of these slime accumulations and their water content are given in Table 11. The economic recovery of even a portion of these values would prove a boon to both producers and consumers of phosphate rock and solve in part the problem of slime disposal.
Slimes containing as little as 30% solids (almost the maximum compaction obtained in slime-pond storage even after years) are of a jellylike consistency. They can be handled like soft sticky clay, but any material containing up to 40% of solids will slump or flow under pressure, so that storage in open piles is impracticable.
T ~ B LIv. E SOLIDS I N W.4SHER DISCHARGE A X D SLIalES %
SLIMES(- 150 MESH)STOREDIN OLD T ~ B L11. E ACCUMULATED SLIMEPONDS Moisture Content,
%
20 t o 45 40 t o 60 60 t o 75 75 and over Total
% of Total Tonnage 18.9 28.4
Sggregate Tonnage 23,652,000 35,680,000 56,250.000 9,918,000 125,500,000
44.8
7.9 100,o
TABLE111. SIZE DISTRIBUTION O F FLORID.4 SLIMES Particle Size, h‘licrons Over 44 44/28 28/9 9/3
3/0.3 Under 0 . 3
PEBBLE PHOSPHATE
% of Total Weight (Range of Typical Analyses) 1 to 3 1 to 4 12 to 18 11 t o 14 14 t o 27 44 t o 52
Solids 2 12 35
Water from washer Slimes after settling 8 days Slimes after centrifuging
TABLE T.’.
CHEMICAL .4XD
MINERALOGICAL ANALYSIS %
Ingredient
20.0 21 . o 18.0 20.0
SiOz CaO AlnOs Fe & Loss on ignition Miscellaneous Total
!.5_ 1 . i 12.0 2.8 100.0
--
Apatite Caa(P0ds.F Kaolinike AlzOs 2SiOz.2HzO“ Wavellite‘ 4AlPOa 2AlIOHOr Limonite,’2Fez03.3Hnd Quartz, SiOn Feldspar, KAlSiaOs Dolomite, Cahfg(C0dz Organic (not determined) Miscellaneous Total a
OF
TYPICAL PHOSPHATIC SLIME
PZOS
NATURE OF PHOSPHATE SLIMES
‘Io the average person, the nord “slime” suggests a slippery, viscous, ooze or mud. To the metallurgist, it usually means ultrafine material difficult to dewater and resistant to ordinary methods of beneficiation. Because of the enormous volumes of Rater required to separate the granular material in the phosphate matrix, the slime suspensions discharged from the phosphate washers and flotation plants contain only 1.5 t o 3% of solids. S o t only does the colloidal nature of these solids keep them in suspension for a protracted period, but even after they have been settled or compacted, they still contain such a high percentage of water that no economic method has been devised for separating their mineral values. Table I11 shows that the bulk of the solid material ranges in size from 3 to 0.3 micron. Some idea of the difficulties involved in handling such finely divided material may be gained from Table IT‘, which shows the low percentage of solids in the slime dischaige after settling.
1051
+ 9HaO)
38 0 30 0 15.0 7.0 5.0 1.5
1.0
1.5
1.0 ___
100.0
h more reasonable assumption is that the clay content is montmorillonite
and/oi attapulgite rather than kaolin.
The chemical and approximate mineral analysis of a typical slime (dry basis) is given in Table V, but no reliable data are available showing the chemical composition and the distribution of the minerals in the slime fractions. Such determinations, although difficult t o make on account of the extreme fineness of the material, are of fundamental importance in a systematic investigation designed to recover phosphate and other values from these slimes. SLIME PROBLEM
The handling and reclaiming of phosphatic slime offer a challenging problem. Not only does the disposal of this rejected material destroy the value of large tracts of land beneath unsightly slime ponds, but it involves expense for constructing and maintaining dams or retaining walls to prevent the slime from oveiflowing adjacent areas and polluting streams. The enormous tonnage of phosphate minerals in the slime already accumulated could supply present fertilizer demands foi a t least 10 yeam, provided it could be converted into an economically usable condition. If the slime from current operations were utilized, the life of the Florida pebble fields would be extended by fully 30 % Any proposal for overcoming the slime-disposal problem or recovering mineral values must, of course, offer some economic advantage over the highly developed system of commercially successful operating practices now employed. The percentage of BPL in the discarded slime solids is nearly as high as that in the matrix from which it is derived, since the bairen silica or sand formerly discharged along with the slime is now removed as flotation tailings, leaving the slime fraction proportionally richer in BPL. The phosphorus pentoxide content of this material (14.8%) per ton of dry solids, if converted into finished products such as superphosphate and phosphoric acid, would be worth from $12.50 to
1052
I K D U S T R I A L A N D ENGINEERING CHEMISTRY
529.50. This value might be further augmented by the recovery of a substantial proportion of the alumina ( 2 5 7 0 ) and the not insignificant quantity of UsOs present in the slime. These total pot'ential values far exceed the gross value of the metal extracted from a ton of crude ore at many profitable metal mines. By contrast, the cutoff point of low-grade porphyry copper ore is belor metal cont,ent and even at the historically high price of 30 cents per pound, the groee value of the recoverable refined metal is substantially less than S6.00 per ton of ore milled.
lr0
Vol. 46, No. 5
sible methods for settling and partial dewatering of slime puspensions produced in the treat,nient, of Tennessee phmphatw, illeluding the following basic processes and variations (5). Sedimentation Chemical treatments Dispersants l;locculants Modifiers, designed to delay gel formation or to produce nicre compressible gel Heteropolar organic compounds, to render clay hydrophobic Positively charged inorganic sols-e.g., aluminum or ferric oxide sols Dyes, selectively adsorbed by clay Dissimilar metal powders, galvanic action Agents for removing calcium ion from solution, by precipitation Physical treatments Cltrasoiiic irradiation Freezing
COURTESY C O R O N E T PHOSPPATE CO
Earth Dams or Retaining Walls of Corner of ?iewI> Created Slime-Di3posal Pond Such calculations, homver, have lit,tlc significance until practical methods of recovering phosphate in marketable form are developed. Many operators feel that there is no immediate prospect of improving the handling of the phosphate mat,rix and that extract,ing phosphate values from the slime xould cost considerably more than recovering an equivalent quantity of high-grade phosphate by the present est'ablished system. As most of the slime from current operations is being segregated, it will be available for use if and when economic methods for reclaiming it are developed. In the opinion of the h I X 4 B Panel, there is no pressing need as yet to solve the problem. The question is: How long can we afford to postpone these developments in an industry that has been shoming such a rapid groivth? Manifestly, here is another opportunity for converting debris into dollars, which has been done so frequently by far-sighted technologists. There are four outst,anding obstacles to the utilization of this phosphat,ic slime as it is discarded in the present system of nmhing, screening, and flotation of Florida pebble phosphate:
1. Low percentage of solids in the water containing the dime as discharged from the washer and hydroseparators. 2. Slow settling of these suspended solids. 3. High water content of settled slime. 4. Extreme fineness or colloidal nature of the mineral coastituent's. Unquestionably, these are serious obstacles and the cost of any mineral dressing steps required must be fully offset by additional mineral values and reducing the slime-disposal problem. SETTLING . ~ N D DEWATERING SLIME. The main interest of the phosphate miners has been to minimize the volume of waste, so as to reduce t,he areas needed for slime ponds and to speed up the return of relatively clear process water for reuse. Most of the research work on phosphatic slime has been aimed a t accelerating the settling and compaction of the slime suspensions. The partial dewatering of this colloidlike material through natural settling and compaction is a slow process and no economic scheme to reduce the mater content below 60 to 70% has yet been developed. The Tennessee Valley duthority, which has studied this problem actively over many years, has investigated virtually all poe-
Keighting with sand or fly ash Dehydration, concentrated salt solutions, extraction with et,he: l*:raci~at,ion, to remove dissolred gases Magnetic field, to promote settling Heating matrix or treating vit,h silicone prior to washing, LO prevent, hydrat,ion of the clay Thickening, with slow stirring Rate of stirring Type of stirring element, Preset,tling Chemical treatments Filtration Filter aids Heat Clieniical treatment Centrifuging Chemical treatment Drying High temperature -4mbient temperature Forcing compressed air through suslmisioti 1:lectrical methods Electrophoresis, direct current Electrode spacing Toitage gradient Intermit,tent current C'heniical treatment lcctrophoresis combined with sedimentation lectrophoresis combined vith filtration Alternating current Voltage
Frequency AIagnetically induced current, magnetic field
liy flow
of suspension throii,aii a
The inost promising of these treatincnts was found to he r l o a and controlled stirring. As far as known, this scheme has not I w n lried on a large scale. Thereas accelerated settling and/or improved compaction oi the slime are desirable both from the itandpoint of slime pollti nianagement and because they ma)' speed up the recovery of process iwter, t,hey contribute little to utilization of t,he slime or to storage of the mat,erial in piles instead of ponds. MoreoT-er, rhe niet,hod employed to obtain these immediate objectives m a y iiiake it even more difficult to remove the renlaining water. Before the settled slime is amenable to t,reatments designed t o concentrate or extract its niineral Iralues, cither it must bc resuspended in water to render wet' processes of recovery appiicaMe, or the balance of the water must be removed in order to apply dry methods of concentration. Solar evaporation has been employed successfully for drJ-ing wet slimes in the hard rock phosphate field and preliminary laboratory experiments indicate that it might work on the l a n d pebble slime, although it has not yet been attempted on a, con:mercial scale. To be effective, however, the material would hare t o he spread in thin layers, which would add to the handling c w t ,
May 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
and if wet treatment were contemplated later, it would be exceedingly difficult to repulp it. TREATMENT OF SLIMEIN CIRCUIT.I n order to avoid rehandling the settled slime and reduce the volume of material discharged to waste, it would be advantageous to recover the phosphate values while the slime is still in the plant circuit and the individual minerals are highly dispersed. Ilaseman ( 3 ) of TVA was recently issued a patent for recovering the phosphate values in suspended slimes. The process consists in selectively flocculating the phosphate minerals by means of a starch solution and separating the floes from the aqueous suspension of clay and other nonphosphatic minerals.
COURTESY CORONET PdOSPhATE CO.
Part of Slime-Disposal Pond Phosphate washer in background
The fact that the colloidlike particles in the slime present an enormous surface and require excessive quantities of reagents to coat them has so far discouraged attempts to recover their phosphate values by flotation. T K O general schemes for treating slime suspensions appear worthy of investigation: l o employ froth flotation after filling the pores of the slime particles with some cheap reagent, such as calcium fluoride, thereby reducing the surface which must be filmed with conventional flotation reagents; and to employ an organic liquid having a preferential affinity for the phosphate, leaving the other minerals in the aqueous suspension, and subsequently recovering this liquid for re-use. Before either method can be properly explored, it is desirable to obtain basic information on the character and particle size distribution of the minerals in this ultrafine material and how they respond to such treatments. D R YSEPARATIOX OF SLIMES.A patented process ( 3 ) for separating the slime fraction from the matrix prior to treating the coarser material by the conventional wet method is described by Greene. This process entails drying the entire phosphate matrix (containing up to 25% of moisture), removing the coarse phosphate pebbles by crushing and screening, differentially grinding the balance of the material, and recovering the dust, or slime fraction, by air separation. Whereas preliminary removal of the slime in dry form would reduce materially the quantity of water required at the washer and flotation plants, the drying of the original matrix and the redrying of the coarse rock and flotation concentrates would add materially to the cost of preparing high-grade marketable phosphate. The economic practicability of such a scheme, therefore, is contingent upon the commercial utilization of the phosphate slime or the products derived from it. POTENTIAL USES FOR PHOSPHATIC SLIME
The potential uses for phosphatic slimes may be grouped under two broad headings:
1053
1. Those in which the slimes are employed directly.
2. Those requiring beneficiation of the slime or the extraction
of marketable ingredients.
Several of these end UECS have been investigated; others have rweived little or no attention. No one of the proposed uses, even if practicable, would consume more than a relatively small proportion of the current production of slime, but in the aggregate the tonnage utilized might be substantial. DIRECTUSE OF SLIMES. - i t least five end uses for separated phosphatic slimes suggest themselve- :
1. Periodic treatment of local pasture lands. orange groves, strawberry land, and vegetable gardens with a water suspension of the slimes. 2. Mixing of the dried slime with other fertilizer materials as a filler and source of hosphorus pentoxide. 3. Production o r a lightweight concrete aggregate. 4. Mixing of the dried slime with rubber and linoleum stock as a rEinforcer and filler, 5 . Mixing of the dried slime Rith high-grade flotation concentrates (to act as a binder and flux) in preparing a phosphate charge for the electric furnace.
IRRIGATIOK TITH SLIMESUSPE~SIOKS. During the Tinter and spring, when it is desired to force the fruit and vegetable crops in Florida, the rainfall is very light. Moreover, the bulk of the Florida soils are not only acid but deficient in mineral nutrients and so sandy that they have a low moisture-retaining capacity. Irrigation with fairly thick suspensions of phosphatic slime and subsequent cultivation would furniqh needed water, improve soil texture, and supply phosphorus pentoxide plant food. Such phosphatic slime would have to compete with finely ground phosphate rock. Whereas the phosphate particles in the slime would be in a much finer qtate of division than ordinary ground phosphate rock, the Iattei contains twice as much phosphorus pentoxide.
COURTESY CORONET PHOSPIIATE C O ,
Hydroseparator, I50 Feet i n Diameter From hydroseparator phosphatic slims is discharged into disposal ponds in background
This use for slimes, moreover, would be seasonal and probably confined to areas within easy access of the phosphate mines. The actual cost of pumping slime is relatively low, but the investment in pipelines and the expense of moving them to new locations would be heavy. Although there is evidence that application of slime suspensions on pasture lands is beneficial, further field experiments are needed to prove the economic value of such treatments. DRIEDSLIMEAS A FERTILIZER FILLER.Mixed fertilizers are sold on the basis of definite formulas with a guarantee that they contain minimum quantities of available nitrogen (N), phosphoric
1054
INDUSTRIAL AND ENGINEERING CHEMISTRY
acid (P&), and potash ( K 2 0 ) . Some inert material or conditioner is usually added, which frequently contains no plant food ingredient. Dried phosphatic s l i m has been suggested as a filler for mixed fertilizer. Its high moisture-absorption capacity would improve the physical condition and it carries additional phosphorus pentoxide which should enhance the value of the final product. However, unless some credit is allowed for the relatively insoluble phosphorus pentoxide: this use of dried slime would he uneconomical.
COURTESY CORONET PHOSPHATE CO.
Discharge of Slime into Disposal Ponds Immense Iboded area and dTinp trees
If the wet slimes could be made to react with ordinary or concentrated ("triple") superphosphate so as to convert the acid salt, CaHd(P04)2, to an intermediate compound, CaHPOd, the value of the product would be much enhanced. LIGHTWEIGHT AGGREGATE FOR COXCRETE.The growing market for precast building units and lightweight concrete has created an enormous demand for lightweight aggregates and t,he increasing use of powdered coal or oil as a fuel for large steam-generating plants has reduced the supply of cinders or clinkers commonly used for this purpose. The Bureau of Mines ha,? investigated clays, shales, and waste slat,e with a view to producing bloated products suitable for lightweight aggregate ( 1 ) . ,Imong the raw materials tested was phosphatic waste from the slime ponds of the American Cyanamid Co. near Brewster, Fla. The dried material was moistened xt-ith water, pugged, and extruded through a 1-inch die, and the extruded product was dried and crushed, and finally calcined in a rotary kiln a t 1020" to 1050' C. The lightweight aggregate obtained had a rodded bulk density of 40 pounds per cubic foot and was described as haring "a ver>-favorable strength-weight ratio." These experiments \yere conducted with a relatively dry charge (approximately 12% moisture). Slime containing even as little as 50% moisture would require fully four times morr fuel oil (85 gallom) per ton of produc.1. At 4.5 cents per gallon, the coat of calcining such materials is estimated at $3.83 per ton of product, or from one third to one half the probable market value. A simpler procedure and a partial de\vatering of the slime by means other t,han fuel would he necessary to manufacture lightwight aggregate a t a profit. Employment of the slime to makc aggregate fails to utilize the phosphorus pentoxide content. However, the main objection is economic. Concrete block manufacturers would be satisfied with a product weighing 50 pounds per cubic foot, which can be made more economicallg from local clays. A 30-pound product probably could be made from phosphatic slimes, but there is no incentive to pay a much higher price for a lighter product.
Vol. 46, No. 5
Finely divided minerals RUBBERCONDITIONER AND FILLERS. play an important role as fillers and reinforcing materials in a number of industrial products. Phosphatic slimes should atlequately meet specifications ralling for a product of extreme fineness. I n the compounding of rubber prior to vulcanization, the most important reinforcing material is carbon black, which increases the tensile strength, modulus, and resistance to abrasion and tear of rubber. The use of certain types of clay (chiefly kaolin) for this purpose has grown to enormous proportions. In 1950, 273,700 tone of clay were employed in the manufacture of rubber products and the price of pulverized kaolin for fillcr ranged from %13.00to Sli.00 per ton. 90information is available as to the suitability of phosphatic dimes for rubber fabrication, but, investigation of possible niarltet8 in the rubber industry appears worth while. Eecause of their extreme fine particle size and complete freedom from grit, phosphatic slimes may also serve as a filler for linoleum, oilcloth, plastics, asphalt, wrapping paper, and the cheaper grades of paint, n-here color is not important. PHOSPHATE CHARGEFOR ELECTRIC FURNACES. The electric: furnace process for the nianufact'ure of phosphorus and phosphoric acid has created a demand for certain low-gradc phosphates whirh were virtually valueless 25 to 30 years ago. If high-gr:a.de phosphate rock is employed in this process, silica (as siliceous grarel) must be added t,o facilitate decomposition of the rock and obtain a free-flowing slag. Therefore, lo\y-grtidcL phosphates having a high silica content may often bc mow cconomically employed than high-grade rock, here tranuportation cost of the raw material is not a rontrolling factor. At present, three electric furnace plants manufact,ure pho+ phorus in Florida, all located close t o the phosphate mines. Their total estimated annual capacity is 30,000 tons of elemental phoaphorus, equivalc~rit to 200,000 toms of high-grade phosphate rock.
itOlPl'OH ~ ~ I L I H G S
3665 Tons
Pk9SVPAT'C
SLIME
2372TOhS
N iH GPAOt iHPf POCK
2963
TOhS
Figure 2.
Utilization of Three Fractions Contained i n 100 Tons of Florida Phosphate Matrix
Various blends of fractions used as charges for electric furnace
For efficient operation of an electric furnace, the phosphate charge should be in lump form, and hence run-of-mine phosphat,es, disintegrated rock, and flotation concentrates must be noduliaed, sintered, or briquetted before they are suitable for use. The physical nature of the Florida phosphatic slimes is such that they vould serve as an excellent binder for the fine granular flotation concentrates. Moreover, addition of these slimes ~vould not only furnish a large proportion of the silica required for fluxing purposes but viould also utilize their phosphate content,. Table T-I s h o m the composition of a typical phosphate matrix,
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1954
the high-grade recovered pebble phosphate, and the waste products rejected in current mining operations. Table VI1 compares a typical electric furnace charge (No. 1) made up of high-grade rock and silica pebbles with three blends having approximately the same chemical composition but consisting of mixtures of highgrade pebble phosphate, phosphatic slimes, and flotation tailings. This comparison is also shown diagrammatically in Figure 2.
TABLEVI.
COMPOSITION O F FLORIDA PEBBLE PHOSPHATE .4ND 15‘ASTE hfATERI.4LS
PERCENTAGE
PZOS
CaO 9iOz Fez08 A1208 $02
r
HzO or loss on ignition hliscellaneous
17.6 23 5 45.0
2.8 7.4 0.6
1 .i
i.b
34.0 45.0 5.0 3.6 4.0 2.0 3.0
20.0 18.0 21.0 5.0 20.0
1.5
12.0 2.8
2.0
...
1.2
22.7 20.4 23 9 7.3 22.7 ,.. 0.9
90 0 0 3 0 8
...
...
2.1
4 0 5 0
,..
0.9
Whereas the proportions of these three constituents would have to be adjusted for optimum furnace operations, on the basis of the tentative 45:40:15 blend given in Table VI1 a company could ut,ilize 78% of its slime and 27% of its flotation tailings in the preparation of feed for phosphorus electric furnaces. Assuming that alumina can be subst’ituted for silica, pound for pound, in the slag, the 50:50 blend of concentrat’esand slime would be workable and would permit even more of the slimes to be utilized. The alternative blend consisting of washed pebble and raw matrix shown in the last column of Table VI1 would bring about only a minor saving in fine phosphate, because the bulk of the matrix would still have to go through the washer. POTENTIAL PRODUCTS FROM SLIMES. Excluding possible byproducts, products that may be derived from phosphatic slime by beneficiation and extraction processes may be listed as:
1. High-grade, finely divided phosphates for direct appliration to the field. 2. Superphosphate for fertilizer purposes. 3. Phosphoric acid and by-products. 4. Concentrated phosphate for fertilizer purposes. HIGH-GRADE, FINELY DIVIDED PHOSPHATES. If the slimes could be enriched in phosphate by simple beneficiation and then dried and pulverized, their competitive position relative to finely ground raw rock would be greatly improved. Even after most of the clay minerals and quartz are removed, the product should retain much of its value as a soil builder and conditioner. SUPERPHOSPHATE. Assuming that it were possible economically to increase the phosphorus pentoxide content of the dried slimes to 30% instead of 20y0 phosphorus pentoxide, they would afford desirable raw material for the production of superphosphate. The finely divided condition of these phosphatic slimes, and the consequently enormous surface exposed to chemical action, should result in more effective decomposition of the phosphate minerals, thereby yielding available phosphorus pentoxide with a smaller acid consumption than is normally required. The steps involved in the manufacture of this basic fertilizer product are described by Waggaman (4).The possible objections t o the use of slime for this purpose are: the high percentages of iron and alumina, .considered objectionable in the manufacture of ordinary superphosphate, and the presence of substantial quantities of other impurities which would consume sulfuric acid without increasing the value of the product. PHOSPHORIC ACIDAND BY-PRODUCTS. Because of the semicolloidal condition of the phosphatic slimes, their conversion into phosphoric acid by “wet processes” would involve difficulties in
1055
removing insoluble residues. Moreover, the phosphoric acid thus produced probably would be badly contaminated with iron and aluminum salts, the removal of which is costly and involves the loss of phosphorus pentoxide values ( 4 ) . The suitability of such acid for the manufacture of sodium phosphates and their derivatives appears questionable. However, should such acid contain an appreciable amount of uranium recoverable by methods developed by the Atomic Energy Commission, the value of this by-product might offset the additional processing steps involved. The acid, after separation of the uranium, could be used directly for agricultural purposes or in concentrated fertilizer materials. The ultrafine filter cake might find a market as a filler and the recovery of metallurgical grade alumina would afford another by-product. COXCENTRATED FERTILIZERS. Concentrated superphosphate is manufactured by treating phosphate rock with suffirient phosphoric acid to convert subqtantially all of the phosphorus pentoxide into monocalcium or mater-soluble phosphate ( 4 ) . ils the phosphoric acid produced from phosphatic slimes would contain substantial quantities of dissolved impurities, the concentrated superphosphate manufactured from it would not be as high grade as that derived from purer acid. To what extent this would lorver the agricultural value of the product can be determined only by actual experimentation. The impure phosphoric acid derived from slimes might be neutralized partially with ammonia, to produce ammonium phosphates for fertilizer use. Here again, the substantial quantities of iron and aluminum salts in the phosphoric acid would invoive economic as well as technical difficultiep.
T 4BLE VII. COXPARISON OF COSVENTIONAL FURNACE CHARGE WITH BIZXDSO F HIGH-GRADE ROCK-4ND LOW-GRADE RISTERIALS AVAILABLE AT FLORIDA PHOSPHATE MINES
Ingredients
P205
CaO SiOp Fez03 AlaOa $02
kq3
Waste Materials Pebble Conventional phosphate (Pebble 45% Pebble Phosphate calcined phosphate slimes 40% 5070, 74%, Silica flotation calcmed Pebble tailings slimes 26%) 15% 50% 25.1 25.0 28.3 33.2 28.5 32.7 23.3 29.6 1 4 , 5a 4.5 2.0 5.4 13.3a 10.9 2.8 1.0 0.6 1.0 2.1 1.5 2.4 0.7 1.0 0.5 1 t o 1.12 1 to 1.13 1 to 2.26 None 78.0 87
Phosphate Matrix (Pebble Phosphate 46%,
Phomhate Mitrix 54%) 25.2 33.4 26.3 3.1 5.9
1.2 2.2 I .6 1 t o 1.27 26.0
Silica-lime ratio Slime utilized, 9% Flotation tailings utilized, ? ’& h’one 27.0 None 26.0 Total Si02 plus AlaOa equals 27.8 in this blend u 8 . 32.4% in conventional charge. Equivalent silica-lime ratio calculated on this basis would be 1 to 1.18, against 1.0 t o 1.12 in conventional charge.
CONCLUSIONS
The disposal of phosphatic slime and the recovery of its mineral values warrant intensive study. Although prompt solutions are not yet imperative, steps should be taken t o investigate ways and means for remedying conditions that will grow rapidly more serious. The disposal of this phosphatic slime is chiefly a matter of local concern, but has already caused considerable controversy. The slime ponds may menace nearby agricultural lands, owing to pot entia1 seepage or overflow, and may be a source of pollution to creeks arid rivers. i l s property values advance and tom-ns arid villages expand, the phosphate industry is likely to be subjected t o increasing pressure to do something about the large areas devoted to slime disposal. These areas of land are not only valueless for other purposes, but they constitute unsightly blots on the landscape. Good public
1056
INDUSTRIAL AND ENGINEERING CHEMISTRY
relations policies may require the elimination of such evesores before they become intolerable. Recovering mineral values from waste phosphate slime is of national concern, as it represents a large potential source of an essential fertilizer constituent -48a first step, fundamental studies should be made of the mineral composition of the slimes and distribution of the phosphate and other potential mineral values determined in the subsieve particle size fractions. The basic information thus obtained should serve as a guide in developing ore-dressing methods applicable to the treatment of this niateiial.
Voi. 46, No. 5
LITERATURE CITED
(1) Conley, J. E., et al., U. 9. Bur. Nines. Rspt. Intest. 4401 (Kovem-
ber 1948).
(2) Greene, E. 1%‘. (to Minerals Separation North American Corp.), U. S. Patent 2,571.866 (Oot. 16, 1951). (3) Haseman, J. F., Ibid.. 2,680.303 (1903). (4) TVaggaman, W. H., “Phosphoric Acid, Phosphates, and PhosSOCIETYMonograph phatic Fertilizers,” A M E R I C I N CHEMICAL 34, 2nd ed., S e w York, Reinhold Publishing Corp.. 1952. (5) Walthall, J. H., private communication. (6) Warren, 9. P., unpublished report.
RECEIVED for review July 16, 1953.
ACCEPTEDJanuary 25. 1934.
Equilibrium Temperatures and Compositions behind a Detonation Wave ALEXANDER WEIR, JR., ~ K RICHARD D B. MORRISON Uniuersity of Michigan, 4ircraft Propulsion Laboratory, I’psilnnti, Mich.
T
HE combustion of Hanimable gaseous mixtures may be
divided into two categories. deflagration or “subsonic” combustion and detonation or “supersonic” combustion. The temperatures in the wake of deflagrations approach the adiabatic flame temperatures, inasmuch as the flow terms are usually of negligible importance. On the other hand, detonation waves are accompanied by strong convective flows which account for temperatures in the wake that are greatlyin excess of the adiabatic flame temperature. Information concerning the chemical composition a t temperatures accompanying detonations is lacking. Ordinary methods of chemical analysis are precluded in determining the composition immediately behind detonation waves, inasmuch as these waves may have velocities in excebs of 10,000 feet per second and the temperatures behind them ale on the order of 4000” K. Emission apectroscopy would be of great value in identifying certain components that are present but ~ o u l dnot, to the witers’ knowledge, provide an instantaneous quantitative analysis of all the gases immediately behind the wave. Hence, somewhzt indirect methods must be used t o obtain knowledge of the chemical composition behind a detonation wave The velocities of these supersonic detonation waves in hydrocarbon-oxygen mixtures 15 ere determined by the utilization of shock tube techniques (9). Laffitte and Breton ( 7 ) , Breton ( 2 ) . and Manson (8) reported detonation velocities of acetyleneoxygen and propane-oxygen mixtures but not hexane-oxygen mixtures. For consistency, Morrison’s data (0) \\ere used for the three fuels in the calculations reported herein. These experimentally determined velocities may be used in conjunction with chemical equilibrium constants to determine the temperature (and hence, the chemical equilibrium composition) which must occur t o satisfy the conservation of mass and momentum. These chemical equilibrium compoqltlons are the limiting values which will occur. They will probably be a closer approximation of the true composition than the valueq obtained by stoichiometryi.e , by assuming complete reactions without dissociation. This paper presents the temperatures and equilibrium compositions for detonation in propane-oxygen, hexane-oxygen, and acetylene-ouygen miutures APPLICATION OF MOME\TUhI THEOREM TO DETOli iTIOR WAVES
A Chapman-Jouguet ( S , 6 } detonation wave may be described aa a supersonic wave with heat addition behind it, that is
propagated through a gaseous Hammable mixture with minimum possible velocity consistent with the conservation laws. This paper considers only the case for Chapman-Jouguet detonations. It has been shown that the gases behind a Chapman-Jouguet detonation wave relative to the wave front a t a speed just equal t o their local sonic velocity-Le., the burned gases in a ChapmanJouguet wave-are moving a t a Mach number of 1 relative t o the wave. This fact, together with the conservation of maps and momentum and an equation of state, may be used to describe the changes across a detonation wave (Figure I).
PRESSURE RISE ACROSS D E T O N A ~ W ION ~E T
+ piulz
(Conwrvation of momentum)
PI
Equation of state--i.e., gau Ian-
p =
ideal
Definition of Mach Jfa number for ideal gases
=
P2
+ p3u;
P/T(R,M)
(1) (2
__
=
u / d q )
Combining Equations 2 and 3 we have P I L ~=
PrMa2
(4)
and by substituting Equation 4 in Equation 1, we obtain
(5) However, for a Chapman-Jouguet Detonation, the Mach numhei behind the wave, M a 2 ,i b equal to 1 and the relationship involvipg the bIach number of detonation ( M a ,= Man) is
This relationship makes it possible to calculate the preswre rise across a detonation wave from the experimentally determined Mach numberq of detonation by assuming that the detona-
