DISSOLUTION OF ROCK PHOSPHATES WITH HYDROCHLORIC ACID

DISSOLUTION OF ROCK PHOSPHATES WITH HYDROCHLORIC ACID 

The objective of the section is the complete dissolution of rock phosphate in hydrochloric acid and separation of undissolved solids by producing clear liquor for further process.

  

DISSOLUTION OF ROCK PHOSPHATES

Dissolution section means the dissolved of rock phosphates. Mainly the rock is acidulated by aqueous HCl and one liquid phase is obtained, which can be said to contain the reactant as well as the ultimate product CaCl₂ and Phosphoric acid. And it has a highly ionized solution in which the ion and the solvent water participate in a large number of equilibria. The major ions H⁺, Ca²⁺, Cl^-, PO₄^3- but minor ions are Fe²⁺, Fe³⁺, F^-, SiF₆^2- and some traces constituents such as vanadium, chromium, magnesium etc. The techniques used to dissolve soluble matter from its mixture with an insoluble solid by liquid solvent called leaching or solid extraction. The solid extraction is an operation in which the particular component of the solid is leached out with the help of solvent. Similarly, in process rock phosphates is dissolved by attacking the rock with hydrochloric acid. The soluble component leached out by the hydrochloric acid from rock phosphates. The liquor which is formed is called dissolution liquor. The soluble constituent of rock phosphates is the target of our in product and the inert are solid residue is just a solid waste. the solvent in which the solute has a high solubility easy availability is definitely preferred but here in our plant both factors are satisfied so we chose the hydrochloric acid to dissolved the rock phosphates.

 

As to increase the rate of dissolution, rate of extraction and to avoid quick settling of the the high specific gravity rock phosphates here preferred continuous stirred tank reactor. By agitation of rock and hydrochloric acid the solid liquid mass transfer increases. And 3 continuous stirred tank reactor used in series at a time to increase the residence time of reaction. The solute diffuses from inside the solid into the surrounding solvent. Extract from a leaching stage are essential solid free liquid, called overflow and the wet inert solid having some amount of solvent is the under flow. To reduce the concentration of solute in the liquid portion of the under flow leaching is often accompanied by counter current flow. the combine process produces a final overflow referred to as extract which contain some of the solvent and most of the solute and a final under flow the extracted and leached solid which are wet with almost pure solvent.

The process of formatting of dissolution liquor occurred in four steps

Ø Attack of phosphates rock by hydrochloric acid

Ø Separation of the solid residue by sedimentation

Ø Washing of the solid residue to recover the P2O5dissolved in the retention solution.

Ø Cooling of dissolution liquor

Properties of raw materials:

 

Rock Phosphate

 

Grade                    : 73/75 BPL

P₂O₅                     : 32 to 34 %

CaO                      : 48 to 50 %

Fe₂O₃ + Al₂O₃       : 0.3 to 0.60 %

Cl                         : 0.02 to 0.05%

F                           : 3.50 to 4.00%

SiO                         :1.75 to 3.00 %

CO                         : 3.50 to 5.0 %

As                          : 3 to 5 ppm

Cr                         : 55 to 60 ppm

Organic Matters     : 0.15to 0.25%

Moisture               : 3 to 5 %

Bulk Density         : 1590 -1650 Kg/m³

 

Size Analysis (Mesh)

+4                        : 0.2 to 0.4 %

+100                     : 65 to 80%

+200                     : 15 to 25 %

-200                      : 3 to 6 %

 

Hydrochloric Acid

 

Molecular weight                                  : 36.46

Total acidity as HCl % w/w min.         : 32

Residue on ignition, % w/w max.        : 0.10

Sulphate as H₂SO₄                                  : 0.01

Sulphate as SO₂ ppm, max                     :5

Iron as Fe in PPM max.                        : 5

Free chlorine and bromine as Cl ppm, max : 10

Heavy metal as Pb ppm, max                :5

 

Appearance               : clear, colourless to faintly yellow

Odor                          : Strong, pungent

Density                                                : 1.15 at 20⁰ C

pH                                                       : <1

Boiling point                                       : 84⁰C at 760 mmHg

Freezing point                                     : -52.5⁰ C

Vapor Density (air=1)                        : 1.257

Vapor pressure                                    : 35 mmHg @ 25⁰ C

Seasonal variation in temperature                 : 40 to 45 ⁰ C

Seasonal variation in Concentration             : 30 to 32 %

The rock phosphates from a intermediate silo transferred continuously to reactor with Adjusted rock phosphates on belt conveyor (having installed weigher) according to the plant load and discharged to reactor through the screw conveyor. four reactors one of them should always in standby. As required discharge rock phosphates in two different point so the screw conveyor is suitable for it as it.

 

The reactor having pitched blade agitators. Internal baffles are provided inside the reactor to promote the stability of power drawn by the impeller and to avoid the fluid swirling, thus enhancing mixing.

 

Here specific quantity of hydrochloric acid 32% from caustic soda plant and ~20 % condensate HCl recovery Section or H3PO4 Concentration section is react with rock phosphates to give phosphoric acid calcium chloride and silica compound.

 

The reaction is given bellow. 

 

Ca₃ (PO₄)₂ + 6HCl à  2H₃PO₄ + 3CaCl₂

 

6HF + SiO₂                   H₂SiF₆ + 2 H₂O

 

CaCl₂ + H₂SiF₆                 CaSiF₆ + 2HCl

 

CaF₂+2HCl     2HF + CaCl₂

 

CaCO₃ + 2HCl    CaCl₂ + H₂O + CO₂

 

 

In the above 5 reactions occurred in the reactor and among of them we have we special interest in first reaction that is our main reaction. here either reactive or non-reactive silica present in rock the non-reactive silica compound settle down at the bottom of the rector as a solid waste.

 

Here excess amount of hydrochloric acid is fed to facilate completion of reaction. Some portion of HCl added through the lower side of the rector to avoid the unreacted particle chocking which accumulate bottom of the rector. Some portion HCL added inlower part means counter current of rock feeding to prevent deposition of sludge with unreacted rock also so in the bottom of reactor fluidization motion going on to extract maximums P2O5 as well as to prevent chocking of drain.

 

Among all reactor there is no special variation in concentration, temperature & reaction rate i.e. concentration throughout the reactor and overflow of the rector is constant.

 

Due to dissociation carbonate compound (CaCO₃) present in rock phosphates as carbon dioxide is evolved out. As a result, during the reaction foaming formation is observed. To avoid that foaming the anti-foaming agent added in the reactor drop by drop or as per required. 

Dissolution liquor comprising Phosphoric acid, calcium chloride, hydrochloric acid, water and some amount undissolved suspended solids on the overflow of the reactor is continuously feeding through gravity into the Clarifier. The separation of undissolved solids material has been done in this clarifier called sedimentation thickener. The clarifier having with scrapers rotating around the central axis of circular tanks. Mechanical solids removal devices move as slowly as practical to minimize re suspension of settled solids.  Tanks are sized to give clear liquor an optimal residence time within the tank. in this thickener material are settled down by coagulation using flocking agent flocculants. From the thickener the clear dissolution liquor as the upper part of the thickener enter into next section. 

Here the completely dissolution process is described .