Phosphate Fertilizer Mixtures

The 1500- pound and 9-ton mixtures were made with a well-cured Wil- .... part of triple superphosphate and two parts of finely ground .... In this ope...
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Phosphate Fertilizer Mixtures Chemical Changes and Physical Effects Induced in Mixtures of Triple Superphosphate with Dry a n d Wetted Limestone and Dolomite W. H.MACINTIRE,L. J. HARDIN, AND F.D. OLDHAM The University of Tennessee Agricultural Experiment Station, Knoxville, Tenn. *

With recognition of the economic factors of manufacture and transportation of concentrated phosphates and the several advantages of mixing such materials with either limestone or dolomite a t the point of usage to obtain the basic types of phosphates that are preferable for some soils, laboratory and field mixtures were studied to secure information for guidance in practice. Speed and extent of diphosphate transitions were computed from carbon dioxide evolutions in closed systems and from decreases in the water-soluble PzO, content of larger piles. The radical departure of prewetting the limestone and dolomite was introduced t o accelelvate reaction and

t o expedite the dry mechanical condition that characterizes maximal diphosphate transition, and in particular t o increase the content of readily available magnesia in the dolomite mixtures. The divergence between the mechanical effects of dry and wet admixtures and also between the effects of machine and hand mixing, the elimination of set-up by reworking the mixtures before storage, the conditions found in unconfined and in immediately bagged mixtures, and related factors were considered. It is emphasized that the findings from a maximal quantity of 9 tons are not intended for application t o larger scale commercial operations.

URING recent years the commercial prac-

mizes hygroscopic tendencies and obviates destructive action upon bag containers and the metal of fertilizer drills. The ultimate physical condition of the fertilizer may also be improved. The presence of ammonium compounds in the phosphatic mixtures does not interfere with the use of dolomite in such mixtures, for it was demonstrated by MacIntire and Sanders (4)that, a t ordinary temperatures, ammoniates may be mixed with dolomite without loss of ammonia. A recent contribution by Beeson and Ross (1) showed that this also holds for monoammonium phosphate. The neutralization of the “free” H3P01by the added limestone, or dolomite, also precludes the toxic effect upon seed germination that was noted by Morse (9), an effect attributable to hydrofluoric acid engendered in the soil by reaction between the free acid and the calcium fluoride residue of the incorporated triple superphosphate. When a substantial transition of the water-soluble phosphates to diphosphates is induced in the prior mixing, the tendency of the added P206 to become fixed by the iron and aluminum components in some soils is greatly retarded. Moreover, the citations ( I , 5, 6, 7) indicate that an appreciable supply of the readily available dimagnesium phosphate can be assured when a superphosphate-dolomite mixture is properly made and sufficiently aged prior to incorporation with the soil. But, to assure adequate supplies of nutrient magnesium directly from separate additions of the relatively insoluble dolomite, it is deemed necessary that the dolomite be incorporated with the soil a month or more before sowing. I n the case of light sandy soils a still longer period may be required. The reactivities of limestone and of dolomite in dry mix-

tice of adding dolomite to “standard” superphosphate and its mixtures, and aging them has reached extensive proportions. The greater usage is due primarily to two factors-(a) the growing tendency toward increased content of ammonium sulfate, urea, and other acid-forming nitrogenous materials, and (b) the recognition of the widespread need for nutrient magnesium. The economic importance and agronomic value of dolomite additions was stressed by Parker (IO), Mooers (8), and Tidmore (IS). The difficulties of and need for analytical procedures adapted to the analysis of fertilizers fortified by dolomite were pointed out by MacIntire ( 3 ) . Pierre (11) developed a laboratory method for the determination of potential acidity of ordinary mixtures and the potential basicity of those that carry basic supplements. This analytical method has been accorded favorable reception by the chemists of the fertilizer industry, and is now the basis of collaborative studies by the Association of Official Agricultural Chemists and the American Society of Agronomy. This reflects the wide recognition that has been given to the conclusion that the use of commercial fertilizers should result in no increase in soil acidity.

Previous Findings and Conclusions The inclusion of limestone in phosphatic mixtures, and their proper aging, are considered to bring several advantages to the manufacturer and the user. The added limestone or dolomite kills off the “free” phosphoric acid, and thus mini1 I n collaboration with the Division of Chemical Engineering, Tennessee Valley Authority.

711

INDUSTRIAL AND ENGINEERING CHEMISTRY

712

VOL. 28, NO. 6

TABLE I. THE TRANSITION OF WATER-SOLUBLE P20aINTO DI-FORMS IN WETTEDMIXTURES“

-__-

Mixture 1b-----53.5% PaOs and Limestone

-InterveningGrams Days COZ 1 13.75 1 2.45 1 0,106 2 0.134 2 0.103 3 0.108 2 0.024 1 0.016 1 0,010 1 0.009 4 0.011 8 3 0 027

:

-TotalDays 1 2 3 5 7 10

12 13 14 15 19 27 30

Grams

Transition monoto di-,

Cot

%

13.75 16.20 16.30 16.44 16.54 16.65 16.67 16.69 16.70 16.71 16.72

83.0 97.2 98.4 99.2 99.8 100.4 100.6 100.7 100.8 100.8 100.9

16:75

ioi : 1

Mixture 4b 3 8 . 5 % PzOs and Limestone

-Mixture 253.5% PtOs and Dolomite Intervening ,----Total-Grams Days C02 Day8 1 1 8.76 2 1.74 1 2 4 5.37 2 0.49 6 3 9 0.038 11 2 0.016 1 12 0.005 1 13 0.005 1 14 0.005 4 0.011 18 8 0,018 26 4 30 0.006

...

..

Grams COz

,...

38.5% PZOS Mixture and Limestone 3 Transition monot o di-,

%

52.9 63.4 95.8 98.8 99.0 99.1 99.1 99.2 99.2 99.3 99.4 99.4

..

Mixture 5 38.5% Pa06 and Dolomite Transition Intervening --TotalmonoGrams to di-, Grams Days COz % Days COZ 1 3.94 1 3.94 1 2 4.14 8.08 2 4 0.85 8.93 2 6 0.92 9.85 3 9 10.40 0.55 2 11 10.94 0.54 1 12 11.08 0.14 1 13 11.22 0.14 1 14 11.35 0.13 4 18 11.62 0.27 8 26 11.78 0.16 4 30 11.82 0.04

Intervening Grams Days COz 1 9.64 1 0.73 1 0.26 2 0.23 2 0.12 3 0.11 2 0.08 1 0.04 1 0.03 1 0.03 4 0.12 8 0.12 3 0.04

--TotalDays 1 2 3 5 7 10 12 13 14 15 19 27 30

Grams COz 9.64 10.37 10.63 10.86 10.98 11.09 11.17 11.21 11.24 11.27 11.39 11.51 11.55

Transition monot o di-,

%

limestone caused a reversion of a portion of the available phosphoric acid when goods containing it are manufactured and stored -Total-Intervening Grams Grams under factory conditions.” Days CO2 Days COz % The effect of high-calcic dry limestone 1 1 8.7 8.7, 72.9 1 2 1.4 10.1 84.6 admixtures upon the water-soluble P206 4 2 10.69 89.6 0.59 content of triple superphosphates has been 1 5 10.83 90.8 0.14 2 7 0.13 10.96 91.7 dealt with only by Larison ( g ) , and no 10 3 0.14 11.10 93.1 14 4 11.21 0.11 94.2 study of systems of dolomite and triple 11.28 17 3 0.07 94.5 superphosphate has been reported. Using 30 13 0.24 11.52 96.6 0.09 37 11.61 97.3 7 a 6-mesh superphosphate that carried 46 9 0.11 46 11.72 98.5 49 3 0.05 11.77 98.7 per cent available PzOs,he made four mix... .. .. ... tures-1 : 3, 1:2, 1: 1, and 3: I-with higha Mixtures constant, air-dr basis, 100 grams 50-mesh superphosphate and 200 grams 100-mesh calcic 20-mesh limestone and allowed the limestone or dolomite, w e t t e z with 10 per cent water: mixtures 1, 2, 3, and 5 run a t fall period laboratory temperature; mixture 4 was kept a t 30’ C. mixtures “to stand in open containers prob PlOs values given in headings for mixtures 1 t o 5 are from determinations of water-soluble. tected from precipitation but subject to all changes of temperature and humidity during tce spring.” The several P205 iralues were determined by means of the A. 0. A. C. methods, pretures with “standard” superphosphate and the changes that take place during the course of the analysis of the cured mixscribed for unaltered superphosphates, a t the end of 30-, 60-, tures have been reported by MacIntire and Shuey ( 7 ) . The and 90-day periods. No attempt was made, however, to chemistry of PzOstransitions induced by dolomite in both dry establish the speed of the transitions. Larison concluded that, under the imposed conditions, the reversion of available and moistened mixtures and in aqueous suspensions has also P Z Oto ~ citrate-insoluble forms was small and of no commercial been reported by MacIntire and Shaw (5, 6). These studies significance. with mixtures of small size indicated that no appreciable change in P206“availability” is to be expected in mixtures of Objectives superphosphates with dry dolomite, up to inclusions of 33 per Because of the increasing production of the “double,” cent of finished product, aged for reasonable periods and a t ‘[treble,” or “ t r i ~ l e ”suDerDhosDhates and the desirabilitv of ordinary temperatures. Even when the mixtures were sub. * * jected t o repeated wet“cutting” them to ordinary PZO5concentratings, u n d e r laboratory tions before their incorporation with the soil, conditions, there occurred a study was made to ascertain the speed, nature, and extent of PZOstransitions induced by no appreciable increase in several admixtures of limestone and of dolocitrate-insoluble PzOjcontent during a 13-month mite in the laboratory and also under outdoor period (6). I t was pointed conditions that would represent the mixing out, however, that this is operations and storage a t the place of usage. A particular objective was to induce speedily not true of h i g h - c a l c i c limestones (6, 7). In a the maximal formation of dimagnesium phosphate, with attainment of optimal ultimate recent paper P. M. Shuey (12) reported results obtained from admixtures of FIGURE 1. SPEEDAND EXTENT OF TRANSITION dolomite with two green O F WATER-SOLUBLE P205 TO DIPHOSPHATE and two dried s t a n d a r d FORMS INDUCED BY DRYAND MOISTENED ADsuperphosphates. T h e MIXTURES, AS MEASURED BY EVOLUTION OF CARBON DIOXIDE UNDER CONTINUOUS ASPIRATIOK mean of his sixteen an9 . Moistened limestone and high-grade triple superalyses of mixtures cured phosphate d u r i n g a p e r i o d of 3.5 B . Moistened dolomite and high-grade triple superphosphate months showed an actual C . Moistened limestone and standard triple superphosphate increase of 0.066 per cent D. Moistened dolomite and standard triple superphosin citrate-insoluble Pz05. phate C ’ . Same as C with constant temperature of 30’ C. Kevertheless, Shuey conE . Dry dolomite and high-grade triple Superphosphate F . Dry dolomite and standard triple superphosphate cluded that “the dolomitic r-

Transition monot o di-,

JUNE, 1936

INDUSTRIAL AND ENGINEERING CHEMISTRY

physical condition, by expediting the reaction between the water-soluble PzOe content of the superphosphate and the magnesium carbonate content of the dolomite. This objective prompted the radical departure of thoroughly prewetting the dolomite and limestone before mixing with the triple superphosphate.

Laboratory Mixtures i n Closed Systems The speed and extent of the PzO6 transitions induced by admixtures of wetted limestone and of dolomite were first recorded by determinations of the continuously aspirated evolutions of carbon dioxide as in previous studies with standard superphosphate ('7). Supplemental analyses of the end products obtained from each limestone and dolomite admixture with two types of triple superphosphates were made when the evolution of carbon dioxide had practically ceased. One superphosphate was essentially a monocalcium phosphate , per that contained 56 per cent total and available P z O ~53.5 cent being water-soluble. The other was made by acidulation of phosphate rock with 74 per cent phosphoric acid, and contained 48 per cent total and 38.5 per cent water-soluble PzOS. Subsequently, a number of admixtures were made in larger quantities-50 pounds, 300 pounds, 1500 pounds, and 9 tons -with periodic analyses by the official methods. The 1500pound and 9-ton mixtures were made with a well-cured Wilson Dam product, and each triple superphosphate was apparently free of hydrofluoric acid. The piles of the large mixtures were later sacked and stored in piled bags. The carbon dioxide data of Table I and the curves of Figure 1 were obtained from 300-gram mixtures of one part of the 50mesh superphosphates with two parts of either limestone or dolomite of 100-mesh fineness, the aspirated carbon dioxide being absorbed in properly protected ascarite bulbs. The data show that the wetted limestone admixtures reacted very rapidly, the minimal carbon dioxide evolution for the first 24 hours being 73 per cent of the theoretical diphosphate equivalent. Each of the wetted limestone mixtures had undergone a Pz06transition of 90 per cent or more within the first three days. There was no great variation in the speed of the carbon dioxide evolutions from the limestone admixtures that stood at laboratory temperatures and those kept a t a constant of 30" C. In both speed and ultimate transitions the values from the more concentrated product of 53.5 per cent water-soluble Pz06content exceeded the corresponding values obtained from the mixtures of the 38.5 per cent superphosphate. The computations for Pz06transitions were made on the basis of the equation, CaH1(PO&

713

half of each of these was registered by the free PzOs within the first five days of contact. The speeds of transitions found for these two dry mixtures are shown graphically, however, along with the same values for the five wetted mixtures, in the curves of Figure 1. The data of Table I indicate that each wetted system came to a pseudoequilibrium when the free H3P04 and practically all of the monocalcium phosphate had been converted to diphosphates, without indications of the formation of any triphosphates. This indication was verified when the mixtures were subjected to analysis by official methods a t the conclusion of the several aspiration periods. The data thus obtained, together with the balances between initial weights and respective losses of carbon dioxide and water, are given in Table 11. The small differences between the total initial and final weights of the mixtures on the air-dry basis are accounted for by the change of the hygroscopic moisture of the superphosphate and the added water to water of constitution, as an offset to the loss induced by evolved carbon dioxide. The currents of carbon-dioxide-free air aspirated through the closed systems to sweep out the evolved carbon dioxide were first washed through columns of water and hence were always more humid than the outside atmosphere. This balance between loss of carbon dioxide and water fixation, or hydration, is in accord with previous observations as to the behavior of mixtures of standard superphosphate with smaller quantities of limestone, and particularly those of dolomite ( 7 ) . TABLE11. CHANGES IN WEIGHT OF MIXTURES IN CLOSED SYSTEMS, AND P 2 0 6 TRANSITION DURINQ PERIODS OF COZ EVOLUTION Exptl. Wetted Mixture5

43% 43% 53% 53% PzOs and PsOr and PlOa and PzOr and

\ stone Weight of mixture, grams Initial 319.0 300.2 Final Moisture content, %: Initial 5.96 Final 0.76 Loss in weipht, grams 18.80 Loss due tTCOz. grams 1 6 . 7 5 HzO actuallv lost: Grams 2.05 0.64 16.77 1.10

mite

stone

319.5 299.6

318.9 305.2

stone 319.5 303.1

43%

hob and mite 319.1 304.3

6.10 0.24 19.90 16.47

5.93 2.12 13.70 11.54

6.10 1.50 16.40 11.75

6.00 2.07 14.80 11.80

3.43 1.07

2.16 0.68

4.65 1.46

3.00 0.94

16.77 1.45

12.07 1.38

12.10 1.20

12.07 1.45

0.98 0.98 Trace Trace 0.98 0.98 0.93 Trace 0.98 Trace 5 Constant dry quantities 100 grams. 50-mesh superphos hate and 200 grams. 100-mesh limestone'or dolomite, the limestone and golomite being wetted with 10 per cent water.

+ CaC08 = 2CaHP04 f COZ f HzO

as though the water-soluble P206content of each triple superphosphate were attributable solely to CaH4(P04)2. When the data of the only mixture (KO. 1) that gave a total carbon dioxide evolution in excess of the theoretical quantity involved in such a postulation were corrected for the double value of the free H3P04 present in the more concentrated superphosphate used in that mixture, the indicated formation of dicalcium phosphate was close to 100 per cent, and no transition beyond diphosphate equivalent was recorded. The transitions registered by the wetted dolomite mixtures lagged slightly behind those registered by the wetted limestone mixtures; but the final joint dicalcium and dimagnesium phosphate values for the dolomite mixture closely approximated those found for the dicalcium engendered in the limestone mixtures. The determined carbon dioxide values and transitions registered simultaneously by the two superphosphates in their exceedingly dry mixtures with bone-dry dolomite are not given in Table I, since both values were less than 8 per cent a t the end of 50 days. Approximately one-

Workroom Mixtures Although the data of Tables I and I1 can be taken as indicative of the acceleration in speed of the Pz06transitions induced by the prewetting of the limestone and the dolomite, the mixtures were not sufficiently bulky to register the physical effects that might be expected in large piles, where the generated heat is dissipated less rapidly. The mixtures of Table 111,approximately 50 pounds each in the air-dry basis, were made with 1 part of 2-mm. triple superphosphate and 2.5 parts of high-calcic limestone of two finenesses, 100-mesh and 2-mm. In the wetted mixtures the dry limestone was first given an addition of 9 per cent water before its intimate admixture with the triple superphosphate. Both the dry and wetted mixtures were then permitted to stand uncovered, well-tamped in cardboard containers for a period of 50 days, after which the several moisture contents were found to be practically identical. The wetted 100-mesh limestone had caused a 90 per cent transition, or nine times the transition found for the dry parallel. In the dry mixtures the coarser

INDUSTRIAL AND ENGINEERING CHEMISTRY

7 14

VOL. 28, NO. 6

OF P20, FROM WATER-SOLUBLE TO DI-FORMS, AND PHYSICAL CONDITION OF WELL-PACKED MIXTURES TABLE 111. TRANSITION AFTER 50 DAYS"

Final Water-Sol. PaOs, % Transition, Treatment Moisture, yo Initial Final % 100-mesh limestone: 0.22 15.3 13.75 10.1 Dry Wettedb 0.20 15.3 1.50 90.2 2-mm. limestone: 0.20 15.3 10.63 30.5 Dry Wettedb 0.25 15.3 2.00 86.9 0 Mixtures of 50 pounds each stored in well-tamped heavy cardboard containers. b Moisture addition of 9 per cent.

TABLE Iv.

CHEMICAL AND PHYSICAL CONDITION O F 100-POUND BAQS O F PILED TRIPLE SrPERPHOSPHATES"

Physical Condition No set-up or caking. very dry product No set-up; slight ciking or granulation, loose, dry, ashlike No set-up! caking, or granulation: dry loom roduot Slight caking into loose porous, easily crushe$lumps

ADMIXTURES O F LIMESTONE AND DOLOMITE WITH Loss

Triple Superphosphates Watersol. PzOs No. 51.5 P-296

Admixture Dolomite

P-297

51.5

Limestone

1.78

0.59

0.16

65.8

P-298

43.4

Dolomite

6.00

2.00

0.13

64.5

P-299

43.4

Limestone

6.00

2.00

0.20

63.1

-Moisture Content-Super--Mixtures----. Residual phosAfter CaCOs phate Initial 52 days Equivalent 63.0 0.15 0.59 1.78

limestone induced a Pz05 transition three times the value found for the 100-mesh dry material. This behavior is attributed to the greater mechanical drying effect induced initially when 2 parts of the exceedingly fine dry limestone were mixed with 1 part of the air-dry superphosphate. But this effect was offset by the 9 per cent addition of water, and the 100-mesh wetted limestone induced the greater PZOStransition. In each case the physical condition of the aged products of this bulk was good, as noted in the last column of Table 111. No mixtures involving coarser dolomite were used, since related work had shown that the low solubility of the larger separates makes inadvisable the use of a ground dolomite that is not made up of larger proportions of relatively fine material. In the next series two superphosphates were mixed with dry limestone and dolomite, 100 pounds of triple superphosphate with 200 pounds of 20-mesh limestone and parallels with 35-mesh dolomite. The 20-mesh limestone contained 65 per cent 100-mesh material, and 80 per cent of the dolomite was of a fineness less than 100-mesh. Each pile of 300 pounds was then put into three sacks of 100 pounds each and piled three sacks deep. After the piled sacks had stood for 52 days, samples of approximately 2 quarts each were removed for analysis. The results for the 52-day period are given in Table IV. At the end of the 52-day period, the maximal loss in the weights of the mixtures was 2 per cent; only one mixture contained as much as 0.2 per cent moisture; the residual carbonate values did not differ materially; the greatest decrease in per cent of water-soluble Pz05was 3.5, and there was no increase in citrate-insolubIe P205,and no set-up. The unused excess of each sample, processed for analysis, was placed in a Mason jar and well tamped, and 42 days later was subjected to the second series of analyses given in the brackets of Table IV. In spite of their very low moisture content, the ground samples showed further decrease in water-soluble PzO6. This was especially true of the limestone mixtures, which underwent not only a material chemical change, but also a set-up. Since the dolomite mixtures did not undergo such extensive chemical changes subsequent to

--

In Wei ht,

-

PnOa Content, Per CenWater- CitrateTotal* so1.C insol. Available None 18.60 18.60 14.25 (18.67) (12.69) (0.5) 14.00 None 19.40 . 19.40 (18.77) (1.25) (0.09) 14.25 . 0.49 16.36 16.85 (17.10) (13.63) (0.64) (16.46) 11.00 1.85 15.66 17.50 (17.34) (1.63) (0.63) (16.71)

L%.

and Per Cent 1.0 1.5 1.0 2.0

Physical Condition of Sacked Mixtures Noset-up No set-upd Noset-up Set-up hmdd Noset-up No set-upd Noset-up Set-up very harda

their being ground, they demonstrated no physical change after being ground and stored.

Fifteen-Hundred-Pound Mixtures A definite set-up of bagged mixtures, machine-mixed, of one part of triple superphosphate and two parts of finely ground dry dolomite had been noted in larger scale operations at Wilson Dam. A slight lumping effect was also encountered when very finely ground bone-dry dolomite was used in some handmade mixtures of considerable size that had been stored in bins. The lumps that occurred in the latter mixtures were readily disintegrated, however, by a stroke from the back of a shovel. It appeared that the slight cementing effect was due primarily, if not solely, to the reaction between the dolomite and the free acid content of the triple superphosphate. It seemed probable that a second mixing shortly after this initial binding tendency would preclude further mechanical effect. T o ascertain the procedure that would circumvent the undesirable physical effect that characterized the dolomite mixtures, the following experiment was conducted : Six 1500-pound mixtures, each of 500 pounds of triple superphosphate and 1000 pounds of dolomite, were made by handmixing. Six corresponding mixtures of superphosphate and limestone were made simultaneously. The dolomite was the same 35-mesh bone-dry product (52 per cent calcium carbonate and 37 per cent magnesium carbonate content) and the limestone was the 20-mesh) 99 per cent calcium carbonate product; both were used in the mixtures of the earlier experiments. Three of the six dolomite mixtures and three of the six limestone units were mixed dry on a concrete floor and left in the 1500-pound piles, unconfined and uncovered. In the other three mixtures of dolomite and in the other three of limestone, both ground materials were well wetted by additions of 100 pounds of water before the additions of the superphosphate. A t the end of the first 24-hour period, one of each of the dry and the wetted mixtures of both dolomite and limestone was sampled and analyzed. Each pile was then remixed by shovels, and 200 pounds of the 24-hour mixture was placed in two 100-pound sacks, the remaining 1300 pounds being left in the pile for 6 weeks without further disturbance, except for sampling at the end of 48 hours, 6 days, and 6 weeks. Another series was handled in the same manner, except that the first sampling and reworking were made at the end of 48 hours, and the third series was handled likewise at the end of 6 days. All

JUNE, 1936

INDUSTRIAL AND ENGINEERING CHEMISTRY

I

[

*

"3

ouaoc

3

0331 3330 3303

2

-mom

moo" 3 0 - m .

.

1

.

.

715

mmmm

....

mom-

.

e

3

4

5

0

WE€/e

FIGURE2 . PROGRESSIVE DECREASEOF WATER-SOLUBLE PzOa IN 15oo-POUND MIXTURES O F TRIPLESUPERPHOSPH.4TE AND LIMESTONE OR DOLOMITE of the twelve piles were again sampled and analyzed at the end of a total period of 6 weeks, as shown in Table V.

..

Considering first the chemical changes, it is apparent that the transition induced by the limestone was more rapid than that induced by the dolomite in both the dry and the wetted mixtures and for all of the four periods of curing. There w&s no positive decrease in available PZOs, or increase in citrateinsoluble P206in the 24-hour, 48-hour, and 1-week periods, and only small variations in those two values for the 6-week period. In each of the twelve mixtures the major decreases in water-soluble Pz06values were attained during the 48-hour period, the decreases beyond that period being positive but not extensive. The speed of the disappearance of the waterfrom the wetted mixtures a t the end of 24 hours soluble P205 was decidedly greater than the corresponding change in the dry mixtures. The progressive changes in the water-soluble PzOj contents of two dry mixtures and the two wetted mixtures are graphed in Figure 2 . The subsequent mixings after the initial mixing of each series did not register material accelerations in the PzOR transitions in the several piles, all of which came rather quickly to a very dry state. This was especially true of t h e dolomite series, in which the water of constitution is a t t h e mean ratio of 1 mole of PO1 to 5 of water in the formation of substantially equal amounts of the diphosphates of calcium and magnesium, as against a ratio of 1 to 4 in the formation of only the dicalcium phosphate in the limestone mixtures. The primary objective, however, was to obviate the previously noted tendency of the dry dolomite mixtures to undergo caking or lumping, when the mixtures were either bagged or confined in bins immediately after the machinemixing operations. As noted, it was postulated that the relatively rapid initial reaction of free H3P04was the cause of this phenomenon and that an early reworking of the exceedingly dry dolomite mixtures would prevent any further caking tendency. When the reworked piles were confined in bags and stored for several months, no further physical change was noted. In contrast to the procedure of permitting the wetted mixtures to react, expand, and dissipate heat and moisture in un-

7 16

INDUSTRIAL AND ENGINEERING CHEMISTRY

VOL. 28, NO. G

Table VI11 All of t h e f r e e a c i d w a s neutralized in these wetted mixtures, and no increase in citrate-insoluble Pz06 occurred. Although the transitions effected by 2-to-1 mixtures of superphosphate with limestone and with dolomite were only 49 and 45 per cent, respectively, of the theoretical values as against the 88 and 81 per cent transitions obtained in the 9-ton mixtures of one part of triple superphosphate with two parts of limestone or of dolomite, the total amounts of Pzo5 changed from water-soluble forms were greater for the 2to-1 mixtures. At the end of 6 days, the physical condition of each 2-to-1 mixture was decidedly inferior to that of the corresponding l-to2 mixture. After the 9-ton lot of the 2FIUURE 3. MIXINQEQUIPMENT AND LOCALE USEDI?; MAKINGTHE T TON to-1 limestone mixture had stood for 6 MIXTURES days in a bin about 9 X 12 feet, and a t a depth of 3 to 5 feet from front to back, the confined piles, another series of wetted mixtures was made mass was in a decidedly hard state, although not so hard as the and the mixtures were bagged immediately. The immedicorresponding dolomite mixture. After the 6 days of curing, ately bagged, wetted phosphatic mixtures with both dolomite the densities of both mixtures were such as to necessitate the and limestone underwent a hard set-up. With this exception, vigorous use of a hand pick to disrupt the piles and effect all of the wetted 1500-pound mixtures could be used for drilltheir removal to the concrete mixer for reworking. This reing without any subsequent handling other than an agitation of working reduced about three-fourths of the material to a any slightly compacted lumps, or a screening of the entire bulk. fineness suitable for drilling, and the remainder required crushing by blows with the back of a shovel and screening. The Nine-Ton Mixtures mechanical condition of these piles was so unsatisfactory that The next step was to carry out the mixing of triple supermixtures of two parts of triple superphosphate with one part phosphate with the wetted dolomite and wetted limestone in of either wetted limestone or wetted dolomite are not recom9-ton lots by means of a concrete mixer. I n this operation a mended. 400-pound charge of dolomite, or of limestone, was placed in Readily Available Magnesium i n Dolomite the apron of the mixer and 40 pounds of water were added and Mixtures stirred into the limestone. Laterjt was found that the better procedure was to introduce the water and then the dolomite In previous work, the amount of dimagnesium phosphate or the limestone. Two hundred pounds of the triple superpresent in the cured superphosphate-dolomite mixtures was phosphate was then added to the surface of the wetted dolocomputed on the basis of the evolved carbon dioxide equivamite or the wetted limestone. The damp 640-pound charge lence, accounted for jointly by the carbonates of calcium and was then dumped into the rotating mixing drum and mixed magnesium substantially in the ratio of their occurrence in the for 2 minutes before delivery into wheelbarrows for condolomitic rock. Analytical methods to differentiate between veyance to bins for storage and curing. With a working the total magnesium content and those fractions attributable crew of six men, 5 to 6 tons of mixtures per hour were handled to the undecomposed magnesium carbonate content of doloin this way by means of any ordinary 1-yard mixing machine mite supplements and to engendered dimagnesium phosphate of the type illustrated in Figure 3. have not been perfected, although such procedures are under At the end of a curing period of 6 days the analyses of study. The amount of magnesia present in the readily soluble form of dimagnesium phosphate in a cured mixture of one Table VI were obtained. Transitions of approximately 88 and 81 per cent were attained by the admixtures of limestone part of Wilsom Dam triple superphosphate and two parts of a and dolomite, respectively, the two types of limestone being wetted Knox dolomite was determined as follows: One-gram the same in composition and fineness as in the previous 1500charges of the phosphate-dolomite mixture were leached with pound mixtures. These Pzo5 transitions of 88 and 81 per cent 250 ml. of water, as in the A. 0. A. C. method. The residue for the wetted limestone and dolomite mixtures are in agreefrom each leaching was then subjected to 30-minute intermitment with the corresponding values of 89 and 79 per cent that tent agitation by hand in 100 ml. of cold 2 per cent acetic acid were found for the 1500-pound wetted mixtures after 6 days. in a closed 250-cc. Erlenmeyer, with continuous aspiration. The physical condition of the exteriors of the 9-ton piles The residues were then filtered. One pair of residues was was good after the 6-day period of storage in wooden bins. used for the determination of total Pz06,and another pair for the determination of citrate-insoluble Pz06. The 2 per cent But the greater compaction and conservation of heat of reaction resulted in a lumping, or caking, of the mixtures a t the acetic acid filtrates were used for the determination of dissolved lime, magnesia, and PzOb. centers of the piles. This lumping was readily overcome, howThe results of Table VI11 show that the 2 per cent acetic ever, either by breaking down the lumps by striking them with acid failed to effect a measurable decomposition of the resithe back of a shovel and screening, or by again running the mixtures through the concrete mixer.. dues of this particular dolomite during the 30-minute period Because of a request to determine the feasibility of making of hand agitation and aspiration. Hence the full value of a mixture of approximately 30 per cent available Pz06con2.58 per cent magnesia found in the acetic acid filtrates repretent, similar 9-ton mixing operations were carried out in the sents the amount of magnesia that was present in the form of same concrete mixer by mixing two parts of triple superphosthe readily available dimagnesium phosphate. The prephate with one part of wetted limestone or wetted dolomite. scribed procedure served its purpose in its use with this parThe analyses of the two products thus obtained are given in ticular dolomitic mixture. If this technic should prove suit~

INDUSTRIAL AND ENGINEERING CHEMISTRY

JUNE, 1936

717

DRYMIXING. Mix 1 part of superphosphate and 2 parts of either dry limestone or dry dolo2 PARTS WETTEDLIMESTONE OR DOLOMITE mite in quantities up to 10 tons, -. Per Cent TransiPer Cent Per Cent PzOj either by hand or by means of Material Moisture Total Water-sol, Citrate-insol. Available Free tion to Di-forms* a concrete mixer. Permit the T. 9 . P. 4.15 46.20 33.63 3 45 41.75 1 29 mixed piles to stand unconfined T. 9 . P. + dolomite 3.57 14.45 2.12 1.25 13.20 0 0 81 1 T. S. P. + limestone 4.22 14.60 1.32 1.08 13.52 0.0 88 2 for 48 hours and remix; then the mixtures may be bagged or stored a Mixtures made in a concrete mixer limestone and dolomite wetted to 10 per cent moisture content. in bins, or they may remain unb Gain in total water and loss of carLon dioxide taken as offsets. confined in piles of 1 to 3 tons until used. This operation produces very dry mixtures, in which IN T TON MIXTURES OF 2 PARTS TRIPLE SUPERPHOSPHATE WITH TABLEV I I . Pz06TRANSITIONS t h e chemical changes are due 1 PART WETTEDLIMESTONE OR DOLOMITE principally to the killing off of . Per Cent Transition Per Cent P206,Air-Dry Basis Per Cent the free acid, with a minimum Mixtures Moisture Total Water-sol. Citrate-insol. Available to Di-forms* of further change. Control” 3.28 30.16 21.54 2.44 27,72 T . S.P. + limestone 2.77 29.60 11.00 2.60 27.00 48:9 WETMIXING. For hand mixT . 9. P. + dolomite 3.05 28.60 11.75 2.54 26.06 45.4 ing, spread 1 ton of dry limea Values computed as tao-thirds of the analytical d a t a on the triple superphosphate. stone, or dry dolomite, upon a b Gain in total water and loss of carbon dioxide taken as offsets. concrete or wooden floor. Add and mix into the limestone or dolomite, 200 pounds (24 gallons) TABLEV I I I . DETERMINATION OF MAGNESIUM PRESENT AS of water. In lieu of either weighing or measuring the volume of DIMAGXESIUM P H O S P H a T E IN A CURED MIXTURE O F TRIPLE water to be added, the limespne or dolomite may be wetted to a SUPERPHOSPHATE AND WETTED DOLOMITE” thoroughly damp, but not soupy,” condition. To the ton of wetted limestone or dolomite, add 1000 pounds of triple superPer Cent phosphate and mix thoroughly. Where larger quantities of the None COz evolved from agitated suspension in 2% acetic acid basic products are desired, the mixtures should be made and piled, 14 80 Total PlOa in original sample 1 20 Water-sol. PzOs leached prior t o acetic acid digestion unconfined, in separate piles of 1 ton and up to 3 tons. 3.50 Total PzOs in residue from acetic acid digestion When using a concrete mixer that is provided with an “apron,” 2.40 Citrate-sol. PzOs in residue from acetic acid digestion the size of the batch is determined, of course, by the capacity of 1.10 Citrate-insol. PsOs in residue from acetic acid digestion 9.25 the apron. First introduce the water into the apron and then PzOh in acetic acid extract 9.08 CaO in acetic acid extract add and, if necessary, use a hoe to mix the water into the weighed 2.58 MgO in acetic acid extract charge of dry limestone or dolomite. Then add the superphosa Measured by the amount of magnesium present in the water-insoluble phate t o give the same 1-to-2 proportion specified for hand mixresidue of a leached superphosphate-dolomite mixture. ing, and dump the contents of the apron into the revolving drum and mix for 2 minutes, or longer, before delivery into wheelbarrows and thence in piles of 1 to 3 tons. Permit the mixtures to stand unconfined for 48 hours, and then turn or rework the piles, as in hand-mixing of concrete. The reworked piles may able for all types of dolomitic supplements, without involving then be bagged or stored in small bins for reasonable periods, correction for liberated carbon dioxide, i t would represent a without danger of hardening. This operation eradicates all free quick procedure for the determination of the magnesium acid and transforms about 85 to 90 per cent of the water-soluble present as the dimagnesium phosphate in dolomite-superPZOSto diphosphates. When dolomite is used in 1-to-2 mixtures the amount of magnesium present in a readily available form will phosphate mixtures. This point will be studied further be equivalent to about 2.5 per cent magnesia. with other types of dolomite of variant calcite content. TABLE VI.

Pzo6TRANSITIONS IN 9-TONMIXTURES” OF

1 PART TRIPLE SGPERPHOSPHATE WITH

I

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Practical Applications The present study is intended as a contribution to the chemistry of the mixing of triple superphosphates with limestone and with dolomite at the point of usage. The findings are not intended to be applicable to large-scale commercial operations. The agronomic considerations of the practice of making separate incorporations of liming materials and concentrated superphosphates, and the alternative of single incorporation of the aged phosphate-limestone or phosphatedolomite mixtures are not within the scope of the present paper. It is recognized, however, that a substantial saving can be effected in long-haul transportation of plant nutrients in concentrated forms, and that for some soils and conditions it is deemed advisable to effect a dilution of the acidic concentrates to basic mixtures of the standard PZOsconcentrations by the use of supplements of either limestone or dolomite. There is also the problem of assuring an immediate and economical supply of readily available magnesium, which, in many cases, is now supplied through the use of watersoluble magnesium salts, rather than from the somewhat slow disintegration of dolomite incorporations in the lighter types of soil. The following directions are based upon studies with mixtures made both in the laboratory and under practical conditions. It is emphasized that the extent of the chemical changes and the physical effects that take place in mixtures, made either by hand or in a concrete mixer, differ in degree from the corresponding results obtained when the mixtures are intimately ground :

Literature Cited (1) Beeson, K. C., and Ross, W. H., IND.ENQ.CHEX., 26, 992 (1934). (2) Larison, E. L., Am. Fertilizer, 73, 548 (1930). (3) MacIntire, W. H., J. Assoc. Oficial Agr. Chem., 16, 589 (1933). (4) MacIntire, W. H., and Sanders, K. B., J . Am. Soc. Agron., 20, 764 (1928). (5) MacIntire, W. H., and Shaw, W. M., IXD.ESG. CHEX., 24, 1401 (1932). (6) MacIntire, W. H., and Shaw, W. M., J. Am. SOC.Agron., 26, 656 (1934). (7) MacIntire, W. H., and Shuey, G. A., IND.ENQ.CHEX.,24,933 (1932). (8) Mooers, C. A , , Tenn. Agr. Expt. Sta., Circ. 34 (1931). (9) Morse, H. H., Soil Science, 39, 177-97 (1935). (10) Parker, F. W., Am. Fertilizer, 76, 2, 13 (1932). (11) Pierre, W. H., IND.ENG.CHEM.,Anal. Ed., 5, 229 (1933). (12) Shuey, P. M., Am. FertiEizer, 82, 3, 30 (1935). (13) Tidmore, J. W., and Simmons, C. F., Ala. Agr. Expt. Sta., Circ. 67 (1934).

RECEIVED March 20, 1936.

GOLD DREDGE OPERATISG BELOW S C M P T E R ORE., I N POWDER R I \ E R VILLEI