Granulation of Powdered Concentrated Phosphorus Fertilizers
Abstract Generated abstract
This paper evaluates the granulation behavior of a newly synthesized concentrated phosphate fertilizer composed mainly of calcium, magnesium, iron, and aluminum metaphosphates with calcium pyrophosphate. Using moisture capacity measurements, the authors calculate a lump-forming index of 0.9 and an optimum working moisture content of about 23.3 percent, then test these estimates in a laboratory pan granulator. The experiments show that fractional water addition improves particle-size distribution, yielding predominantly 1 to 2 mm granules, while partial dissolution and recrystallization within the granules promote self-setting and strength development. The authors propose a simple process scheme involving premixing, pan granulation, and low-temperature fluidized-bed drying, while noting the need for pilot-scale optimization.
Full Text
GRANULATION OF POWDERED CONCENTRATED PHOSPHATE FERTILIZERS
V. M. VITYUTIN, V. A. TROFIMOV, T. G. LEONTYEVA, V. V. TIKHONOV
(Presented by the scientific seminar of the Department of General Chemical Technology)
Recently, at the Institute of Chemical Sciences of the Academy of Sciences of the Kazakh SSR, in the laboratory of mineral fertilizers, a concentrated phosphate fertilizer was synthesized. The material is a comparatively finely dispersed gray powder consisting of a mixture of metaphosphates of calcium, magnesium, iron, and aluminum, and calcium pyrophosphate. The approximate composition of the sample is:
\[ \begin{aligned} \mathrm{P_2O_5} &— 57.0\%;\\ \mathrm{CaO} &— 25.0\%;\\ \mathrm{MgO} &— 1.5\%;\\ (\mathrm{Fe},\ \mathrm{Al})_2\mathrm{O_3} &— 4.5\%;\\ \mathrm{SiO_2} &— 12.0\%. \end{aligned} \]
The use of such a dispersed fertilizer is expedient only in granulated form; in this case the technical requirements for the granulate are as follows:
Sieve composition of the granulate
\[ \begin{aligned} -3 + 2\ \text{mm} &\le 5\%;\\ -2 + 1\ \text{mm} &\ge 80\%;\\ -1 + 0.5\ \text{mm} &\le 10\%;\\ -0.5\ \text{mm} &\le 5\%. \end{aligned} \]
The granules must be mechanically strong for transportation and thermally stable up to a temperature of \(150^\circ\mathrm{C}\).
To develop a rational technology for fertilizer granulation, an assessment was made of the powder’s ability to form lumps. The ability of dispersed materials to undergo wet aggregation is determined by the relation:
\[ K = \frac{W_{\mathrm{MMB}}}{W_{\mathrm{MKB}} - W_{\mathrm{MMB}}} \]
where \(K\) is the index of the material’s lump-forming ability.
The maximum molecular moisture capacity \((W_{\mathrm{MMB}})\) is determined by the “moisture-capacious media” method; the maximum capillary moisture capacity is determined by the method of capillary saturation of a column of freely poured dispersed material with water.
Experimental Part
Below are the averaged results of determining the characteristic moisture capacities of phosphorite fertilizer:
\[ W_{\mathrm{mmv}} — 21.0\% \quad \text{(relative to dry material);} \]
\[ W_{\mathrm{mkv}} — 45.0\% \quad » \]
Corresponding to these moisture capacities, the lumping index of the fertilizer is:
\[ K=\frac{W_{\mathrm{mmv}}}{W_{\mathrm{mkv}}-W_{\mathrm{mmv}}}=0.9 \text{ units.} \]
The fertilizer belongs to easily lumped materials and will granulate well both in pan and in drum granulators.
The optimum moisture content of the powder before lumping is calculated according to the equation we proposed:
\[ W_{\mathrm{opt}}^{p}=\frac{W_{\mathrm{mmv}}}{K}=23.3\%. \]
The analytical moisture content of the material is
\[ W^{a}=2\%, \]
and thus the addition of water to the powder before lumping is equal to 21.2% (relative to dry weight).
The correctness of calculating the optimum value of the working moisture content before lumping is illustrated by the data in Table 1. Lumping was carried out in a laboratory disk granulator with a diameter of 300 mm, at a peripheral speed of 1 m/sec and a disk inclination angle of 45°.
Table 1
Results of lumping in a granulator \(d=0.3\) m
| No. | \(W_p\), % | Granulation time, min | Granulate characteristic |
|---|---|---|---|
| 1 | 25.0 | 1 | Very large pellets (up to 15 mm), sintering in the granulator |
| 2 | 23.5 | 2 | Large granules (up to 10 mm), sintering during prolonged rolling |
| 3 | 22.5 | 2 | Normal in size, but comparatively weak granules |
| 4 | 22.5 | 6 | Normal in size, strong granules |
| 5 | 21.5 | 6 | Granule formation does not occur |
The sieve composition of the granulate is determined to a significant extent by the nature of water dosing. Thus, preliminary complete moistening of the powder before lumping to the optimum moisture content (experiment 4, Table 1) leads to the formation of granulate of the following composition:
\[ -3+2\ \text{mm} — 29.3\%; \]
\[ -2+1\ \text{mm} — 64.1\%; \]
\[ -1\ \text{mm} — 6.6\%. \]
A granulate more uniform in sieve composition is obtained with fractional moistening of the powder. Thus, if the initial powder is moisten—
is brought before pelletizing to 21%, and the remaining water (up to the optimum moisture value) is fed through a nozzle into the granulator onto the material being rolled, then the composition of the granulate is expressed as:
\[ \begin{aligned} -3+2\ \text{mm} &— 14.1\%;\\ -2+1\ \text{mm} &— 81.1\%;\\ -1+\ \text{mm} &— 4.8\%. \end{aligned} \]
Thus, the screen composition of the granulate is easily regulated, and obtaining granules of a specified size under industrial conditions presents no difficulty.
A very important feature of phosphate fertilizer as an object of granulation is its capacity for partial dissolution with subsequent crystallization of the saturated solution inside the granules. This circumstance has an extremely favorable effect on the dynamics of the processes of granule formation and strengthening.
It has been established that at room temperature, in an excess of distilled water, about 12% of the powder passes into solution. This leads to a substantial increase in the amount of the liquid phase relative to the insoluble residue at the moment of moistening and promotes rapid granule formation.
The subsequent supersaturation of the intragranular solution causes crystallization and a corresponding decrease in the relative moisture content of the granules, which is favorable for strengthening the structure of the wet granules. A kind of self-setting of the granules occurs. Simple holding of the wet granules in air without heating for 6 hours ensures complete fixation of the structure of the wet granules.
As preliminary experiments have shown, the most expedient method for drying the wet granules should be considered to be their treatment in a fluidized bed in an air stream at a temperature 10–15°C above the dew point of the exhaust gases. Increasing the temperature of the drying gases (air) above 100°C is inadvisable, since in this case the conditions for formation of a strong crystalline structure of the finished granules deteriorate.
The easy granulatability of the powdered fertilizer studied makes it possible to choose the simplest granulation scheme:
Primary
water
↓
Air-dry powder, W ≈ 2%
↓
┌──────────────────────────────────────────────────────────────┐
│ Dosing and mixing in a double-shaft paddle mixer │
└──────────────────────────────────────────────────────────────┘
↓
Initial mixture for granulation, W = 21%
Secondary
water
↓
┌──────────────────────────────────────────────────────────────┐
│ Granulation in a pan granulator │
│ at V_circ = 1 m/sec; α = 45° │
└──────────────────────────────────────────────────────────────┘
↓
Wet granules, d = 1–2 mm, W = 23%
↓
┌──────────────────────────────┐
│ Drying in a fluidized bed │
└──────────────────────────────┘
↓
Dry granules for shipment
Heated
air ───────────────→ Drying in a fluidized bed
t_air = t_d + 10°C ───────────────→ dust–air mixture
↓
Cyclone cleaning
↓
← return dust
Conclusions
-
The concentrated powdered phosphate fertilizer is an easily granulated material.
-
A basic process flow diagram for granulating the fertilizer has been proposed.
-
It is necessary to carry out a detailed pilot-industrial study of the recommended technology in order to establish the optimum operating conditions.