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INDUSTRIAL AND ENGINEERING CHEMISTRY
hoarded in a n out-of-the-way place for a season, a t the end of. which it can be handled through conventional disposal channels. FERTILIZERS PRODUCED The phosphorus content and niodc of use, as regards both crop and location, of the radioactive fertilizers produced in the spring of 1948 are given in Table 11. I n most instances the design of the field experiments required a n amount of inactive fertilizer three to five times t h a t of the radioactive fertilizer. Data for these inactive materials are given in Table 111. ACKNOWLEDGMENT Cooperation of H. J. Baker and Bro. in supplying CuiaQeo phosphate rock, of the Davison Chemical Corporation, Coronet Phosphate Company, and the Tennessee Valley Authority in the preparation of inactive fertilizers required for the control field experiments, and of Isotopes Division, Oak Ridge Operations,
Vol. 41, N5. 7
U. S . Atomic Energy Cornmission in supplying radioactive phosphate on a rigid time schedule and in providing for a n inspection of plant and techniques by competent health physicists is gratefully acknowledged. Thanks are also ext,eiided to J. 0. Hardesty of the Department of Agriculture mho supervised the ammoniation, and t o S. B. Hendricks and TA. A. Dean of t,he depart,nient who supervised t,he monit,oring. LITERATURE CITED (1) Dean, L. 9.,et aZ., Proc. Soil Sck. Soc., 12, 10T-12 (1948). ( 2 ) Dean, L. il., unpublished data. ( 3 ) Morgan, K. Z., Chem. Eng. News, 25, 3i94-8 (1947). (4) Nat. Committee on Radiation Protection, “Safe Handling o l
Radioisotopes,” provisional draft. (5) Nelson, W. L.. et al., Proc. Soil Sci. Soc., 12, 113-18 (1948). (6) Parker, F. F7r., Plant Food J . , 2, No. 2, 4-9, 33 (1948). RECEIVED August 26, 1948. This work was m p p o r t e d in part by a p r ~ x i t from tho Industry Phosphate Refiearch Committee.
An optimum ratio of raw materials exists a t which the total raw material cost per unit of available phosphorus pentoxide i n cured superphosphate will be at a minimum for any given rock and acid cost. The value of the phosphate rock depends not onIy upon its phosphorus content, but also upon the extra acid consumed by impurities which participate i n the reaction. I t is possible to predict the optimum rock-acid ratio by means of a nomograph based o n the phosphorus and carbon dioxide content of the rock. These nomographs are used as a basis for predetermining the most economical ratio of rock t o acid a s well as the total and available phosphorus pentoxide of the finished superphosphate.
atio in Superphosphate MARK SHOELD, E. H. WIGHT, AND VINCENT SAUCHELLI THE DAVISON CHEMICAL CORPORATION, BALTIMORE 3, MD.
B
ECAUSE the superphosphate industry is highly competitive, the margin of profit is relatively small. This matter of profit margin is very important. Management and particularly the superintendent responsible for production are very conscious of costs. Anything t h a t can lower unit costs of production is a favorable survival factor and is eagerly sought for by the producer. The method described here was originally developed by Shoeld and Wight of the Davison research staff and has been found practical and dependable for many years. Everyone COIIcerned with the manufacture of superphosphate has wished for a quick, dependable, simple method of determining in advance certain economic control factors in the process based on the chemical analysis of the phosphate rock ground to a certain mesh size. Rock is bought on specifications. The operator is given an analysis sheet showing the bone phosphate of lime, calcium phosphate (B.P.L.) content or its equivalent as phosphorus pentoxide, the moisture content, and the screen test. Many operators, especially in smaller plants, lacking a chemical control dcpartment are forccd t o follow some rule-of-thumb procedure. I n a n y case the operator mould like to be able to decide quickly first, how t o predict the amount of acid required for the most economical conversion of the phosphorus pentoxide t o a n available form, secondly, how t o predict the analysis of the finished superphos-
phate after a definite aging or curing period. Or, perhaps ht. would like t o know in advance t h e most economical degree of conversion under the conditions of his operation. Many operators have learned through experience how t o judgc. the acid requirement factor. Many do a good job, as long as the basic factors remain fairly uniform. If, however, a new factor suddenly is injected into the procedure, i t may trip up the operator badly because he has no way of analyzing it quickly and adjusting his process t o provide for it. T h a t is where the man who understands each factor and how it operates in his scheme of things has the advantage. From the authors’ studies, i t is known t h a t a n optiniurn mtio of raw materials exists a t which the total raw material cost per unit of available phosphorus pentoxide in the cured superphosphate will be at a minimum for each set of rock and acid cost items. T h e value of a rock depends upon its phosphorus pentoxide content and whether or not extra acid has t o be consumed by impurities of the rock which, participating in the reaction, also consume acid. In working out in the laboratory the method which was later t o be used by the plant superintendent, i t was obvious t h a t the laboratory procedure should adhere closely t o plant practice. It was imperative that the investigators should maintain controlletr conditions for a considerable number of conversions.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1949
1335
where i t meets the line of “97% acidulation.” From t h a t point, draw a vertical line downward until i t meets the “acidconsumption” scale. This point reads 1085 pounds of acid, and this is the amount t o be used for 1200 pounds of rock. A second nomograph based on these studies, shown in Figure 2, is used t o determine the total and available phosphorus pentoxide in a superphosphate if the percentage of conversion, and the phosphorus pentoxide and carbon dioxide contents of the rock are known. An example follows: The rock analyzes 33% phosphorus pentoxide and 1.5% carbon dioxide; the conversion required is 9i’700 and ; the curing time is 30 days. As in the first chart, start at the intersection of the abscissa and ordinate in the side rectangle giving the composition of the rock. From this intersection draw a line t o the right horizontally, until it meets the heavy vertical line in the main chart marked, 10% “moisture.” From this point, follow the oblique Figure 1. Acid Consumptions v s . Rock Composition and line t o the right until it cuts the vertical line designating 97y0 Conversion acidulation. At this point draw a horizontal line t o the exRun-of-pile 30-day curing, 130’ F. acid temperature treme right t o the phosphorus pentoxide percentage scale. Where it intersects the scale one reads 19.77,, which gives the total phos horus pentoxide. T o get the available phosphorus pentoxide, fo8ow the same oblique line t o the vertical phosThe laboratory control method was developed from 35 different phorus pentoxide scale a t the right; then from the point where i t procedures which checked closely with the practice at Davison’s intersects t h a t line, follow the reverse oblique line t o the vertiCurtis Bay plant. This method may not be suitable for other cal line designating 97% acidulation. From the intersection with this acidulation line, draw a horizontal line t o the right until plants and other rocks without appropriate modifications. Havit intersects the phosphorus pentoxide scale; the intersection ing developed a satisfactory laboratory procedure, the next step gives the answer-namely, 19.1% ’ phosphorus pentoxide. was t o determine whether or not it worked with different rocks. Figures 3 and 4 show a number of curves, each of which repreIt is possible for rock from different sources t o differ in composisents variations in rock and acid prices and variations in the tion and for the sulfuric acid used to be varied in strength and carbon dioxide content of the rock. These studies reveal the imtemperature. portance of the carbon dioxide factor in the rock. T h e literaI n the study six Florida pebble rocks were used varying in ture does not give much information on this particular item. In phosphorus pentoxide content from 31 to 34% and in carbon the graph the cost of the rock phosphate is on a per short ton basis dioxide content from 1.5 t o 4%. Six different rock-acid ratios ground ready for processing. The acid price is per short ton, were used. T h e curing periods were varied: 2, 14, 30, and a final basis 50’ B6. curing period of 60 days. T h e rocks used in the acidulation were grourld t o a standard: Through Through Through Through
60 80 100 200
mesh mesh mesh mesh
98%
92 % 85 t o 90% 58 t o 607,
10 PER CENT MOISTURE.+
Chemical analyses were made according t o standard Association of Official Agricultural Chemists’ analytical methods. The carbon dioxide content was determined accurately by means of the absorption method; this method must a t all times be used in this procedure, because of its high degree of accuracy. It was found from the laboratory procedure study that two factors had t o be constantly recognized-the temperature employed is very important and if excess water is removed too quickly in the reaction, conversion is substantially inhibited. NOMOGRAPH DEVELOPED
As a result of these studies it was possible t o predetermine the optimum rock-acid ratio. By means of a nomograph, based upon the phosphorus pentoxide and carbon dioxide content of the rock, the percentage of conversion after curing for any rockacid ratio is quickly determined. T h e phosphorus pentoxide and the carbon dioxide determinations must be known accurately. This nomograph, illustrated in Figure 1, is based on a 50’ B6. acid at 60” F. per 1200 pounds of rock. This follows the plant practice of reporting. Actually the concentration of the acid was 55 O B6. and the temperature, 130” F. T h e acid consumed by the calcium carbonate and the silicon tetrafluoride was deducted. T h e curing time for the superphosphate was 30 days. Any other curing period could be used in making a similar chart. The nomograph is read as follows: Find the amount of acid required for a 97% conversion of a rock containing 33% phosphorus pentoxide and 1.5% carbon dioxide. Follow the ordinate and abscissa corresponding to 33y0 phosphorus pentoxide and 1.5% carbon dioxide, respectively, i n the small left-hand rectangle which gives the rock composition. Locate the point where they meet. From this point draw a horizontal line t o the right, so t h a t it cuts across the oblique lmes for acidulation. Locate the point
PER CENT
Figure 2.
ACIDULATION
Total and Available Phosphorus Pentoxide vg. Rock Composition and Conversion Constant moisture
All the curves have a minimum point which lies around 98% conversion. When the ratio of rock t o acid price decreases-that is, when the acid is relatively more expensive than the rock--the tendency of the curve is t o flatten, and the most economical acidulating percentage is reduced. Although the lowest point of curve D is at 98% conversion, it cannot be assumed that, from a practical viewpoint, such a point represents the most economical conversion. T h e difference in raw material cost between a 95 and 98% conversion, curve D,is only 0.2 cent per unit available; this would correspond t o 3.2 cents per ton, basis 16% available phosphorus pentoxide. Many operators pride themselves on producing a superphosphate having a minimum insoluble phosphorus pentoxide, say
INDUSTRIAL AND ENGINEERING CHEMISTRY
1336
0.2 or 0.301,. These curves show cleaily t h a l such piide niay be misplaced. I t all depends upon the relative cost of rock and acid. It is possible t o have a 0 2% insoluble material a t a greater net cost than a product with 0.6 or 0.7% insoluble phosphorus pentoxide kd 2
270 8
6 4 2
260 8
0 " " 0
*
4
1
2
z
250
4 t
B
a
6
$
4
+
5
2 240 8
6 4
2 210 8 84
86
88
90
92
94
98
100
Raw Material Cost Conversion
VS.
96
PER CENT A C I D U ~ A T I O N
Figure 3.
Vol. 41, No. 7
able phosphorus pentoxide will be 49.8 pounds. This is then plotted on the chart. The chart represents data for six samples of rock phosphate. The figures from all six rocks for 30-day acidulations are obtained in n similar manner and plotted on the chart. All the data fell on the one curve. Five of the six rocks used in the laboratory were actually converted in the plant. The results of the plant operations when calculated in 8, similar manner also fell on this same curve. If the points from all the different rocks had failed to fall on the same curve, it would have been impossible both to correlat'e the data from the rocks of varying analyses and to prepare the nomographs. HOW NOMOGRAPHS WERE DEVELOPED The nomograph in Figure 1 was developed as follorvs: The objective of this investigation mas t o predetermine the amount of acid consumed by a specific rock based upon its composition (phosphorus pentoxide and carbon dioxide contents) for any definite conversion of t,he phosphorus pentoxide desired. Four factors have to be considered: phosphorus pentoxide content of rock, carbon dioxide content of rock, pounds of 50 BB.acid per 1200 pounds or rock, and per cent conversion. Two other factors perhaps should be considered-namely, ohe acid consumed by the volatilization of t'he fluorine, and the acid consumed by the iron and alumina content. The effect of fluorine is taken into considerat,ion as a function of the present conversion (Figure 6) and is eliminated, therefore, as a separate factor. Iron and alumina probably are present' as phosphates and would be included in the acid required for the acidulation. The acid consumed by the iron and alumina was not corrected for in the master curve (Figure 5) and, therefore, their effect is included in the nomograph whether it is a function of acidulation or is a constant independent of it. 6
30-day curing t i m e for rock-phosphate A = 32 PzOa rock 55 per t o n : 50' B6.acid 54 p e r t o n ; ratio, 1:1.25 B = 32 PzOs rock 54 per t o n ; 50' Be. acid $4 per t o n : ratio, 1: 1.OO C = 32 P10, rock $4 per t o n : 50' BC. acid $5 per t o n ; ratio, 1:0.80 D = 33 P206rock $4.29 per t o n : 50" B6. acid 53 60per t o n : ratio, 1:1.18
4
2 78 0
8 6 .4
Figure 3 brings out the effect of carbon dioxide content upon the raw material cost of the rock. The general shape of all these curves is the same, whether carbon dioxide is present or not. However, it is very evident t h a t the carbon dioxide content may have a significantly more important effect upon the raw material cost than a considerable variation in the percentage of conversion. The calcium carbonate of the rock consumes acid without the production of any available phosphorus pentoxide. For the same phosphorus pentoxide content of rock and the same conversion this cost increase is directly proportional t o the cost of the sulfuric acid. If the sulfuric acid cost is relatively high, and one has the choice betn-een rocks of varying carbon dioxide content, this feature deserves practical consideration. Figure 5, the master curve, represents data also of highly significant value. The time and effort expended on this whole investigation were justified by the development of this one curve alone. In drawing the curve the total acid consumption minus the amount required for the calcium carbonate and for the volatilizat,ion of the silicon tetrafluoride a t a fixed conversion rate is plotted against the per cent conversion of the phosphorus pentoxide. The curve shows that the acid consumption for the t,ricalcium phosphate per unit available phosphorus pentoside is a function of the percentage of conversion, is independent of the compositjon of the rock, and remains substantially constant for 30-day curing a t 49.5 pounds of 50" RB. acid from 65 to 92y0 conversion. From 92% on, the curve becomes a logarit,hmic curve approaching infinity asymptotically as acidulation approaches 100%. I n understanding this master curve an example will give the best explanation. Consider the following specifications: A rock had 31.25% phosphorus pentoxide; 900 pounds of acid; 1200 pounds of rock; conversion 82.3y0. The carbon dioxide content, 2.4%, required 103 pounds of acid; t'he hydrofluosilicic acid required 29 pounds of acid. This left 768 pounds of acid for the conversion of the phosphorus pentoxide. The available units, obtained by multiplying total units of phosphorus pentoxide by the per cent conversion, give 15.4. Thus, the acid consumption per unit of avail-
.2
77 0 0
.8
aN
.6
0 W
$ 4
2
8
.4
2 760
.6
.4 .2
55.0 .E
.6
A .2
54.0
84
92
94
96
98
100
PERCENT ACIDULATION
Figure 4. R a w Material Cost vs. Conversion (1947) 30-day curing t i m e for rock phosphate E = 33 P& rock $9 Der t o n : 50' BB. acid $9 per ton-: ratio,. 1 :1 F = 34 PzOa rock $15 per t o n ; 50' B6. acid $11 per t o n ; ratio, 1:1.36 ~
The preparation of this nomograph was made possible by the fact that the carbon dioxide and the phosphorus pentoxide coxitents of the rock are directly proportional to their acid consumptions as shown in the master curve (Figure 5 ) . Certain other considerations were involved-the curing time had to be constant. In this study the authors assumed it t o be 30 da n. q h e pounds of acid per 1200 pounds of rock were represented o n the abscissa scale. An arbitrary point which corresponded t o a
July 1949
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
1337
Pigure 5
definite c o n v e r s i o n s a y 97%-for a rock of a high carbon dioxide content was located vertically above the corresponding acid consumption figure. From this point a horizontal line was drawn t o the left to form the basis of a rock composition diagram. This diagram was drawn on the basis of increments of rising carbon dioxide along a 45" line drawn t o the right and a 45' line for phosphorus pentoxide with decreasing phosphorus pentoxide sloping to the right. An arbitrary scale was chosen for the phosphorus pentoxide which fixed the scale for the carbon dioxide. All rocks at 97y0 conversion were plotted i n the master curve (Figure 5) and fell on a straight line. All other conversion lines were developed in a similar manner. The acid consumption figures were arrived at easily from the master curve (Figure 5) for the amount consumed by the tricalcium phosphate per unit of available phos horus pentoxide, @us that consumed by the carbon dioxide t h a t for the fluorine (derived from Figure 6). The total acid consumption was recalculated to a basis of 1200 pounds of rock. The second nomograph (Figure 2) was based on the laboratory data for 30-day curing of all the rock samples used. The 10% moisture line refers t o the moisture content of the ex-den superphos hate. W i e n the total phosphorus pentoxide determinations were calculated as indicated on a 2 % moisture basis and plotted against the degree of conversion it was found that for each particular rock the total phosphorus pentoxide content fell on a straight line sloping t o the right and furthermore, that the total phosphorus pentoxide content of rocks of varying composition fell on parallel lines. I n the construction of the chart, a rock composition diagram was developed in the same manner as for the first nomographthat is, by means of choosing, arbitrarily, a suitable scale for phosphorus pentoxide and making the carbon dioxide scale correspond t o it. Figures 1 and 2 are based on hundreds of determinations. The analytical error of a single determination is in the range of 0.15 t o 0.20%. For a 20% total phosphorus pentoxide i t was found t h a t every 1.0% conversion corresponds t o 0.2y0 phosphorus pentoxide. At the Davison Curtis Bay superphosphate plant more than 5,000,000 tons of superphosphate have been produced under the guidance of these two nomographs. During t h a t time only one shipment of rock phosphate showed any irregularity and it was only of minor importance. The average results of hundreds of determinations on silicon tetrafluoride volatilization during conversion (Figure 6) indicated a yield of: (1) 12.5 pounds of hydrofluosilicic acid per ton of 16% run-of-pile superphosphate, basis 16.26% total phosphorus pentoxide at 98% conversion, 30-day curing period; (2) 8.99 pounds of hydrofluosilicic acid per ton of a 16 to 20% superphosphate, basis 20% total phosphorus pentoxide, at 82% conversion, 30-day curing. Reducing these t o a n equivalent basis:
ant!
1. 15.4 pounds of hydrofluosilicic acid per ton of run-of-pile, basis 20% total phosphorus pentoxide. 2. 9.0 pounds of hydrofluosilicic acid per ton of 16 to 20% superphosphate, basis 20% total phosphorus pentoxide.
The amount of 50" BB. acid required per 1200 pounds of 32% phosphorus pentoxide rock for each of the above conversions was found to be: for (1) at 98% conversion, 48 pounds, calculated as follows:. 32/20 X 15.4 X 3.28 X 0.6 ton = 48; for (2) at 82% conversion, 28 pounds, calculated as follows: 32/20 X 9 X 3.28 X 0.6 ton = 28. The volatilization of silicon tetrafluoride a t 82% conversion is 58.5% that a t 98% conversion. The reactions involved are:
3CaF,
+ 3HzSOa+ SiOz = 3CaSOa + HZSiFe + 2H20
.LA
I 1%
-
i5
70
80
-185
L 30
55
100
PER CENT ~ ~ C I D U L I \ T I O(IN N 4 WEEKS)
F i g u r e 6. Volatilization of Silicon T e t r a fluoride d u r i n g Conversion The number of pounds of 50" BB. acid per pound of hydrofluosilicic acid equivalent is obtained as follows:
98 144 X 0.622
= 3.28 1b.of
50' BB. acid per lb. of HzSiFa
Above 92% conversion the additional amount of silicon tetrafluoride evolved is relatively small under the conditions involved. FLUORINE EVOLUTION The evolution of silicon tetrafluoride is a function of the coriversion percentage, strength of acid, and of the temperature. Because the acid strength and temperature a t the plant are constant, it was necessary to consider only the fluorine evolved versus percentage of conversion. Figure 6 represents the volatilization of silicon tetrafluoride during the conversion. It is based upon hundreds of fluorine determinations on rock and ex-den superphosphate samples and upon the results in a recovery plant of relatively high efficiency. I n the figure pounds of 50" BB. sulfuric acid per 1200 pounds of rock are plotted against the percentage of conversion. This method was used because it was found impossible t o evaluate the silicon tetrafluoride evolution on the basis of single determinations or on laboratory conversions. CARBON DIOXIDE VOLATILIZATION
It was found that substantially all the carbon dioxide i n the calcium carbonate present was driven off during conversion, regardless of the degree of conversion. CONCLUSION Without any preconceived ideas, the authors set out to determine if i t were possible to correlate phosphate rocks of variable analyses with certain economic factors so as t o make certain predictions with reasonable precision. On the basis of the various analyses as described it is concluded t h a t it is possible to find this correlation speedily by reference t o the prepared nomographs. On the basis of the phosphorus pentoxide and carbon dioxide composition of the rock i t is possible to predetermine the most economical acid-rock ratio as well as the total and available phosphorus pentoxide content of the finished superphosphate. It is also possible t o determine from these same nomographs the phosphorus pentoxide and carbon dioxide contents of the rock required t o produce any desired run-of-pile analysis as well as to select a rock t h a t will prove t o be the most economical for a particular set of conditions. RECEIVED August 26, 1948.
(End of Symposium)
