APATITE & ROCK PHOSPHATE

Phosphorous, nitrogen and potash are the three main plant nutrients. Phosphorous plays an important role in root development and in the synthesis of protein, fats and carbohydrates. Its deficiency in the soil can be made up by adding natural or manufactured phosphorous bearing minerals / rocks. Apatite and rock phosphate are the two important minerals of phosphorous. Apatite is of two types. i.e., chlor-apatite having a composition 3Ca+3 (PO4) 2CaCl2 and fluor-apatite having a composition 3Ca (PO4) 2CaF2. Apatite is of igneous origin and is found in veins. Rock phosphate is a secondary substance formed due to the accumulation of organic remains like bones and by replacement of limestone, calcite, etc., by phosphoric solution to form a mixture of calcium phosphate. It does not have a definite chemical composition. These minerals are mostly utilized for the manufacture of chemical fertilizers like super phosphate, di-calcium phosphate, triple super phosphate, mono-ammonium phosphate, di-ammonium phosphate and other complex fertilizers. Apatite and rock phosphate are also utilized for the manufacture of phosphoric acid (H3 PO4), which in turn is utilized for the production of pure chemicals like sodium phosphate, mono-calcium phosphate, di-calcium phosphate, etc.

The other important uses of apatite and rock phosphate are in the manufacture of elemental phosphorous and in soil reclamation by direct application to soil in ground form.

Since the commercial exploration of rock phosphate at Jhamarkotra commenced lot of discussions about the specifications of rock phosphate required by phosphoric acid, single super phosphate and elemental phosphorous manufacturers have taken place at various forums. By and large, rock phosphate having the following specifications is acceptable to the consuming industries:

A.Rock Phosphate

1.Wet Process Phosphoric Acid:

P2O5

32% (min.)

SiO2

5% (max.)

CO2

4.5% (max.)

L. O. I.

5.00% (max.)

Fluorine           

4.00% (max.)

Chlorine

150 lpm (max.)

MgO

0.5% (max.)

CaO: P2O5      

1.6%

2.         Elemental Phosphorous:

Phosphate rock of a lower grade can be used for the production of elemental phosphorous. Rock containing as low as 24% P2O5 and as high as 23% SiO2 with 3% iron and 7% alumina can be used. The slag forming component like CaCO3, MgCO3 P2O5, etc. should be as low as possible. The rock should be hard and compact so that it can directly be changed in to the furnace. High content of slag forming impurities will require excessive SiO2 resulting in the dilution of P2O5 content in the charge.

The BIS (IS: 11224 1985) has prescribed the following specifications of elemental phosphorous (Type-I) and phosphoric acid (Type II).

S. No.

Characteristics

Requirements
Type - I          Type - II

i

Total phosphate (As P2O5) percent by mass (min.)

30.00

32.00

ii

Silica (as SiO2) percent by mass (max.)

10.00

5.00

iii

Carbon dioxide (as CO2) percent by mass (max.)           

2.00

3.00

iv

Fluoride (as F) percent by mass (max.)

2.00

4.00

v

Mixed aluminum and iron dioxide (as Al2O3) & Fe2O3 respectively percent by mass

3.00

3.50

vi

Moisture, percent by mass (max.)        

1.50

1.50

vii

Magnesium oxide (as MgO) percent by mass (max.)

0.50

0.50

viii

Chloride (as Cl) percent by mass (max.)

0.015

0.05

ix

Organic matter and combined matter, percent by mass (max.)

2.00

1.50

3.         Single Super Phosphate:           

The P2O5 content in rock phosphate for the manufacture of single super phosphate should have a minimum of 31% P2O5 SiO2 up to 8% can be tolerated. Iron and alumina i.e., R2O3 should not be more than 3.5%. Higher R2O3 may tend to reversion of available P2O5 (water soluble P2O5). Carbonate up to 5% will improve the reactivity of rock phosphate by increasing the reaction temperature and also by making the mass porous.

4.         Direct application of rock phosphate as fertilizer:

The use of rock phosphate for direct application as fertilizer depends on its level of solubility in the acidic soil. This application is dependent up on the structure and chemical composition of the rock. Mineralogical tests should be done to assess the suitability for direct application. It is stated that carbonate radical is responsible for the reactivity of directly applied P2O5 in the rock. According to PPCL the following can be considered as specifications for utilizing rock phosphate as phosphatic fertilizer for direct application in acidic soil.

1.

Absolute citrate solubility index

7% max.

2.

Apatite carbonate ration CO2% / P2O5%

0.035

3.

Origin of rock phosphate

Sedimentary

4.

Mesh size

100

5.

Hydro-oxyle iron in crystal lattice is higher indicating substitution of OH for PO4: H2O content

2%

6.

Grade of rock phosphate powder        

16% P2O5

7.

Iron as Fe2O3  

5%

8.

Ca O : P2O5

1.80

A.     Apatite:          

The specifications of apatite for fertilizer industry are the same as that of rock phosphate discussed above.

Apatite is also applied directly to the soil. According to the present practice in India the average P2O5 content in the ore should be 16%. The iron content should normally be less than 5%; however, the field tests are the more definite criteria for the selection of low-grade apatite for direct application to the soil.

Future trends:

In India most of the existing phosphatic fertilizer plants and phosphoric acid plants were designed on the basis of high-grade imported rock phosphate. The Indian deposits are generally of low-grade. Therefore, the future fertilizer plan s for the plant that may be set up as replacement to the existing ones will have to be designed to accept indigenous rock or a blend of important and indigenous ones. Trends indicate that supplies from world markets are also showing decline in grade. In sure the user industries in future will have to orient them selves to allow higher limitations of deleterious constitute.

Beneficiation of low-grade ores is another forward-looking step. Considerable progress has been achieved in this field and significant success has been achieved. Already beneficiation plants have been sent up by Rajasthan Stage Mineral Development Corporation at Jhamarkotra and HZL at Maton. It appears to be necessary to set up additional plants in other areas. Only the key constituent, namely P2O5 has been considered for grade-wise classification reserves.

Rock

+ 30% P2O5

Useable in manufacture of chemical fertilizer, phosphoric acid and elemental phosphoric.

25 to 30% P2O5

Blendable with + 30% P2O5high-grade for manufacture of chemical fertilizer.

20 to 25% P2O5

Useable for direct application to soil.

11 to 20% P2O5

Beneficiable.

11% P2O5

(conditional resources)

Low not useable are present.

After examining the specifications of the user industries and the future trends of beneficiation the group recommends the following end-use classification of reserves for both apatite and rock phosphate.

Acid & Fertilizer:

P2O5

30% and above

SiO2    

5% (max.)

CO2    

3% (max.)

Fe

4% (max.)

Mixed aluminum and iron oxide

3.5% (max.)

Moisture

1.5% (max.)

MgO

1.5% (max.)

Cl

0.05% (max.)

Organic matter and combined water

1.5% (max.)

Direct application to soil:                       

P2O5

16% (min.)

Fe2O3

5% (max.)

CaO:    P2O5   

1.80

Absolute citrate solubility index

7% (max.)

Beneficiable:
                        P2O5                                                    + 10%

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