I
ALON TALMI, E. R. HERMAN, SIMCHA HAREL, and BENJAMIN PESKIN Fertilizers & Chemicals Ltd., Haifa, Israel
How to predict acid requirement in Superphosphate Manufacture
ACD of a rock depends primarily on its chemical composition, REQUIREMENT
although factors such as required conversion, reactivity, mixing, and curing time will modify it to some extent. Satisfactory plant control has been based on phosphorus pentoxide and carbon dioxide analyses (5). However, any large variation in sesquioxide ( R 2 0 3 ) content can cause serious errors and the method is not applicable to calcined rock. Other formulas ( I , 3 ) based on calcium oxide or phosphorus pentoxide analyses likewise have limited applicability. X formula ( 4 ) has been developed for the theoretical acid requirement of a rock, based on its complete analysis. I n the work reported here, a rapid analytical procedure replaces the tedious complete analysis. Basic Considerations
The quantity of acid required to convert 100 units of rock to superphosphate is given by
+ MgO -+ MnO + + KzO + Fee03 + A1103
Pi06 -4, = A , M
-
- SO$)
49.0 X r X e X H F
rock samples assumes that an ideal superphosphate has all its phosphorus pentoxide, other than that compounded with the sesquioxides, in the form of monocalcium phosphate; thus phosphoric acid (H3P04) donates one equivalent of acid. Sulfur trioxide donates two equivalents and the monoand divalent metals consume acid according to their normal equivalents. In cured superphosphate, R203 forms compounds (4) of the type Ca[H2R(P0,)2], X HzO. Evidence is stronger for iron than for aluminum. If acidulation converts all the phosphorus pentoxide to monocalcium phosphate, followed by a secondary reaction with the sesquioxides, then R i 0 3 consumes two equivalents of acid
+
2Ca(H.~P01)2
R203
Ca[HzR(PO&]2
A , = 98.1 (CaO ?"a20
This simple analytical methocl is useful for control in superphosphate manufacture and for economic evaluation of
(1) (2)
here
A , = corrected acid requirement, 100 X required weight of 100% sulfuric acid per weight of rock A , = acid requirement, uncorrected for fluorine CaO, PzOS,HF, etc. = moles per 100 gram of rock r = fraction of fluorine remaining after acidulation e = effective equivalent of fluorine remaining
In this presentation the rock is regarded as compounded solely of oxides and hydrofluoric acid. The formula
+ H8OI + Cas04 + 3HzO +
Any fluorine escaping either as hydrofluoric acid or silicon tetrafluoride does not affect the acid balance. Fluorine remaining may be present as fluoride, silicofluoride, or perhaps other compounds and will have an effective equivalent of less than one. Silica, whether precipitating or escaping as silicon tetrafluoride, does not require or donate acid. Some metal silicates may remain undecomposed but this causes no error if the rock is analyzed on an acid-soluble basis.
Procedure T o 5 grams (w) of rock are added 20 ml. of water, 30 ml. of hydrochloric acid, and 2 ml. of nitric acid. The mix-
ture is boiled for 5 minutes, cooled, transferred to a 250-ml. volumetric flask, and diluted to volume. Insoluble material does not interfere. -4 10-mi. aliquot is pipetted into a borosilicate dish and evaporated to dryness on a water bath. Several milliliters of water are added, followed by exactly 10 ml. of 0 . 3 sulfuric acid. The mixture is evaporated to dryness and left on the water bath for an additional 30 minutes, then stirred with water, and decanted into a 500-ml. beaker. The crystals are crushed with a small rubber bung held in a glass tube, slurried with water, and decanted. Any large crystals remaining are again crushed and transferred to the beaker. Water is added to a total volume of 300 ml. and the mixture stirred until the sulfate dissolves. One milliliter of mixed indicator (23 mg. of methyl red, 67 mg. of bromocresol green, and 10 mg. of cresol purple dissolved in 50 ml. of ethyl alcohol and diluted to 100 ml.; prepared fresh each month) is added and titrated with 1 ml. of 0.1N sodium hydroxide (carbon dioxide-free). The color changes from red to blue and the end point is a gray color free from any red. The solution is allowed to stand for 10 minutes and further sodium hydroxide added if necessary to restore the color. Fluorine in the rock is determined by a standard procedure. The acid requirement is then A,
=
50 - a 12.26 X W
(3)
A, = A , - 1.52 X '?&fluorheinrock ( 4 ) VOL. 51, NO. 5
MAY 1959
675
Discussion As sulfuric acid does not effect complete attack on the rock, other acids must be used. Nitric acid oxidizes organic matter and ferrous iron. A large excess of hydrochloric acid is used to dissolve the rock and ensure decomposition of the less volatile nitric acid. Removal of volatile acids is virtually complete. This was shown for nitrate by testing with diphenylamine; for chloride and fluoride the solids obtained after evaporating several samples were combined, distilled with perchloric acid, and the distillate tested for halogens. Because hydrofluoric acid attacks the evaporating dish, borosilicate glass should be used to avoid introducing alkali to the solution. The precipitated calcium sulfate must be dissolved completely, for the solid retains acid. The correct pH for the end point was determined by titration using a pH meter. The steepest slope was obtained at p H 4.8 which corresponds with the gray of the indicator. The monosodium phosphate determined in this titration corresponds with monocalcium phosphate in super. Sesquioxides form substantially tribasic salts, and thus have the same acid requirement as in the double salts mentioned previously. The formation of tribasic ferric phosphate is quantitative. Suitable quantities of analytical grade ferric ammonium phosphate were added to dilute solutions of monocalcium phosphate or phosphate rock. To readjust to pH 4.8 after allowing for the ammonia and sulfur trioxide added required 2 equivalents of acid per mole of ferric oxide. The effect of aluminum oxide was tested by dissolving pure aluminum in a known quantity of acid, adding aliquots to phosphate solutions, and titrating as above. Acid requirement of aluminum oxide was found to be about 10% lower than theoretical, indicating the formation of basic phosphates. The difference is not important for the quantities of aluminum normally found in rock. The method was further checked by comparing experimental values of A , with those calculated from complete analyses. In most cases differences of less than 1% were obtained and with the two rocks on which most of the work was done agreement was very close.
The method is very precise. A set of four parallel determinations often give results differing by only 0.27,. As no separations are involved, errors are practically confined to the volumetric manipulations.
Correction for Fluorine The factors, r and e, in Equation 2 will vary somewhat with type of rock and conditions of acidulation. A value of 0.6 was found (4)for e but this appears to be low. Acidulation experiments (2) gave values between 0.67 and 0.93 with a mean of 0.79, based on analyses of water-soluble and water-insoluble fluorine and assuming that those correspond to silicofluorides and fluorides, respectively. M’dilla and Gafsa rocks gave values below the mean, while Morrocco rock, Land Pebble, and Kola phosphates had values consistently above the mean, A value of about 0.75 for r appears normal for most plants producing single superphosphate. The value of 0.59 has therefore been used for the product r X e which results in the factor 1.52 in Equation 4.
Degree of Acidulation of Superphosphate A method is sometimes required for determining the degree of acidulation of superphosphate which is independent of extent of cure. While the procedure described previously may be used, it is preferable to take an 8-gram sample of super and to acidulate with 10 ml. of an 0.1N sulfuric acid. With super only the factor e is required for the fluorine correction. The formulas then become Acid deficiency
D,
= 12.26
X
- a grams H2S01 W
10
per 100 grams super ( 5 ) Corrected acid deficiency
D, = D,
-
2.04 X
%F
Acknowledgment in super
(6)
Four samples of super were tested by conventional method for free acid and water insoluble phosphorus pentoxide and their acid deficiencies calculated. Results compare well with the measured Do values.
The proposed method gives reproducible results which agree with the theo-
DC
676
+0.2 +0.3
-2.3 -2.5
The help and advice of Rolf Wardi are gratefully acknowledged.
Literature Cited (1) Bridger, G. L., T V A Chem. Eng. Rept., No. 5 (1949). (2) Delomenie, H., International Super-
phosphate Manufacturers’ Assoc. Tech-
Conclusions
Correlation between This Method and Conventional Methods on Different Samples of Superphosphate 3.5 4.8 5.1 5.1 Free acid [24-hr. cure] yo 1.9 0.7 1.1 1.2 Water-insoluble PsOS, Yo +0.3 -3.4 -2.9 -2.7 2 (water-insol. PlOb) - free acid P z O ~ HzSO4 deficiency, g./lOO-g. super
retical values calculated from complete analyses. These values are based on the supposition that conversion of the rock to monocalcium phosphate is complete and certain additional assumptions concerning the behavior of fluorine and sesquioxides. Superphosphate contains some free acid and some unattacked rock. From the authors’ experience, these factors cancel out very closely and the measured value of acid requirement corresponds with plant practice. Conditions could arise, however, where this was no longer true and then small discrepancies would appear. The value of the method for plant control or laboratory acidulation experiments is twofold. If an unfamiliar rock is to be used, the measured value will nearly approach the optimum, thus saving much trial and error work. Where, however, rock of similar origin but of slightly differing quality is used daily, adjustment in the acid to rock ratio may be made according to variation in the measured acid requirement. As fluorine content may be assumed substantially constant, measurement of the -4, value only is required. The method is particularly valuable as an aid to the economic evaluation of rock samples obtained during beneficiation studies. The ratio of A, to per cent phosphorus pentoxide in the rock is of fundamental importance because it denotes the quantity of acid required to produce super containing unit quantity of phosphorus pentoxide. This value, R tis termed the acid ratio. Complete evaluation of the rock also requires consideration for handling and transportation costs per unit depending on its phosphorus pentoxide content and such other factors as cost of grinding and processing, grade of super produced, and debit for sesquioxide content.
-2.0 -2.0
-1.9 - 1.6
nical Meetings, September 1955. (3) Fox, E. J., Hill, W. L., IND.ENG. CHEM.44, 1532-6 (1952). (4) Marshall, H. L., Hill, W’. L., Ibid., 32, 1128-35, 1224-32 (1940); 44, 1537-40 (1952). (5) Shoeld, M.,Wight, E. H., Sauchelli, V., Zbid.,41, 1334-7 (1949). RECEIVED for review February 26, 1358 .4CCEPTED July 9, 1958 Joint Technical-Agronomic Conference, International Superphosphate Manufacturers’ Association, Lausanne, September 1956.
INDUSTRIAL AND ENGINEERING CHEMISTRY
