The present invention relates to a process for the production of
ammonium phosphates starting from phosphate rocks. The treatment is known of the
phosphate rocks with strong acids such as, e.g., sulphuric acid, nitric acid,
The most commonly used among these is sulphuric acid, which allows
converting all calcium contained in the phosphate ore into calcium sulphate, which
is then removed by filtering.
The reaction in this case is as follows:
Ca&sub3;(PO&sub4;)&sub2; + 3 H&sub2;SO&sub4;+ 6H&sub2;O → 3 CaSO&sub4;.2 H&sub2;O
+ 2 H&sub3;PO&sub4;
The phosphoric acid solution extracted is then submitted to concentration
and subsequent ammoniation.
From the Czechoslovakian Patent No. 142533, decomposing the phosphate
rocks by means of sulphuric acid and ammonium sulphate in three steps is known
In the first step, for the decomposition sulphuric acid is used,
in the second step ammonium sulphate and/or ammonium bisulphate, in the third step
sulphuric acid is used again.
A process for the production of ammonium phosphates has been now
surprisingly found, starting from phosphate rocks which are decomposed by means
of sulphuric acid and ammonium sulphate under suitable conditions in one reaction
step only, which allows the required amount of treatment agents (sulphuric acid
and ammonium sulphate) to be reduced, at the same time reducing the equipment
The process being the object of the present invention for the production
of ammonium phosphates starting from phosphate rocks, which comprises a step of
grinding of said rocks, a chemical attack with sulphuric acid and ammonium sulphate
and a filtration of the reaction pulp, is characterized in that the attack is carried
out in one single step, under isothermal conditions, at a temperature comprised
within the range of from 70 to 90°C.
The temperature control (the reactions which take place are exothermic)
can be carried out by surface evaporation in vacuo.
The attack of the rocks can be accomplished by means of an equipment
provided with such a stirring device as to secure conditions of temperature and
of concentrations of the chemical species present and of suspended solids as uniform
The main reaction on which said process is based is the following:
Ca&sub3;PO&sub4; + 2 H&sub2;SO&sub4; + (NH&sub4;)&sub2;SO&sub4; + 6 H&sub2;O →
→ 3 CaSO&sub4;.2H&sub2;O + 2NH&sub4;H&sub2;PO&sub4;
The process being the object of the present invention allows replacing
portions of sulphuric acid with ammonium sulphate and saving ammonia compared to
the traditional production process through phosphoric acid and the subsequent
ammoniation thereof. This point is of undoubted financial interest if one thinks
that presently large amounts are available of ammonium sulphate, obtained as byproduct
from some industrial processes(e.g., caprolactam production) or from plants for
the sequestering of sulphur oxides from flue gases.
Under the conditions of the process, dihydrate calcium sulphate crystallizes
off, in an easily separable form, which, together with precipitated impurities
and unreacted rock, constitutes the gypsum, subsequently removed by filtration
and washed with countercurrent water for the recovery of nitrogen and P&sub2;O&sub5;.
The process disclosed allows also operating with concentrations of
P&sub2;O&sub5; in the reaction pulp higher than in the known dihydrate processes
for the production of diluted phosphoric acid with reduced formation of hemihydrate
calcium sulphate (involving filtering difficulties).
This element has a positive incidence on the consumption of steam
in the step of concentration of the solution produced, when this is processed to
Moreover, the adaptability has been experimentally demonstrated of
the proposed method to the processing of phosphate rocks from different sources
and having different compositions (Algerian, Moroccan rocks, and so forth). In
these cases, the treatment yield [(dissolved P&sub2;O&sub5;/insoluble P&sub2;O&sub5;
in the rock)×100] reaches industrially acceptable values (92-95%).
Hereunder some preferable typical parameters of the attack step are
- Granulometry of ground rock: 90% < 200 mesh (0.074 mm);
- Excess of SO&sub4; (from H&sub2;SO&sub4; and (NH&sub4;)&sub2;SO&sub4;): lower
than or equal to 10% relatively to the stoichiometrically equivalent amount of
CaO contained in the rock;
- Residence time of the pulp: 2.5 - 4 hours.
Thanks to the treatment process as above mentioned, a pulp is produced
which, by separation of the gypsums, yields a partially ammoniated filtrate (pH
This filtrate, when the ammoniation i$ completed, by the addition
of anhydrous ammonia, up to the desired level (for MAP, up to pH = 4.5-5), is submitted
to traditional operations of fertilizers industry (concentration of the solution,
The proposed attack process and the subsequent operations can be
executed in continuous.
The invention shall be now better disclosed with the aid of the flow
sheet of attached Figure, showing a preferred embodiment, but which is not to be
considered as limitative of the same invention.
The phosphate rock (1) is suitably ground in a mill (2) and previously
mixed in a pre-mixer (3) with a washing solution (4) from which a mixture (5), which
is sent to the rection equipment (6), is obtained. Sulphuric acid (7), also in
highly concentrated solution, is supplied to the reaction equipment in the point
(8), and is fed by devices of various types (sprayers, and so forth), not shown
in the flow sheet.
Ammonium sulphate (9) is sent to the reaction equipment in the point
(10). When it is available in the solid state, it is previously mixed with the
above mentioned washing solution (4) in a pre-mixer (11). From the top (12) of
the reactor,the vapours evolving from the reaction are removed by means of a vacuum
The reaction pulp (13) is on the contrary first sent into a tank
(14), then filtered on a filter (15) to separate the gypsums (16).
From the pipe (17) the desired solution is drawn. Finally, by the
pipe (18) the recycle solution containing nitrogen and P&sub2;O&sub5;, obtained
by countercurrent washing the gypsums with water (19), is recycled.
As an alternative to the above, the phosphate rock and ammonium sulphate
can be mixed at the same time with the recycled washing solution and the so-obtained
mixture is sent to the reaction equipment.
Examples 1 - 2
The results are reported of treatment tests for two types of phosphate
1) Rock from Algerian source (low grade, high content in impurities);
2) Rock from Moroccan source (h1gh grade, low content in impurities);
using sulphuring acid and ammonium sulphate as the treatment agents.
The attack tests have been carried out in an equipment designed to
accomplish conditions as isothermal as possible, and of concentration of the chemical
species in solution and of the suspended solids as uniform as possible.
The attached Tables A, B, C report, respectively, the compositions
of the phosphate rocks used, the granulometries of the same submitted to the reaction,
and the operational attack parameters.
The filtrates obtained (pH ≃ 2.5) have been subsequently submitted
to the ammoniation completion (pH 4.5) by addition of ammonia. The ammoniated solutions
have been evaporated to dryness, and the salt obtained has been analyzed (Table
* The product denominated "MAP" contains, in addition to the very
mono ammonium phosphate (i.e., NH&sub4;H&sub2;PO&sub4;), further compounds (ammonium
sulphate, and so forth), in minor amounts, wich reduce the grade of the fertilizer
1. Process for the production of ammonium phosphates starting from phosphate
rocks, comprising a step of grinding of said rocks, a chemical treatment with sulphuric
acid and ammonium sulphate and a filtration of the reaction pulp, characterized
in that the chemical treatment is carried out in one single step, under isothermal
conditions, at a temperature comprised within the range of from 70 to 90°.
2. Process according to claim 1, wherein the chemical treatment in carried out
by introducing amounts of sulphate ions, supplied by sulphuric acid and ammonium
sulphate, non higher than 10% relatively to the stoichiometrically equivalent amount
of calcium oxide contained in the rock which is being decomposed.
3. Process according to claim 1, wherein the isothermal conditions are accomplished
by surface evaporation in vacuo of the reaction pulp.
4. Process according to claim 1, wherein the singlestep chemical treatment is
carried out in an equipment provided with a stirring device.
5. Process according to claims 3 and 4,characterized in that it is carried out
with a residence time of the pulp comprised within the range of from 2.5 to 4