DESC:

FORM 2
THE PATENT ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006

COMPLETE SPECIFICATION
(See section 10 and rule 13)

TITLE OF THE INVENTION:
A process for extraction of phosphoric acid from rock phosphate.

APPLICANT:
Grasim Industries Limited, an Indian Company, having address at Aditya Birla Centre, B-Wing, 2nd Floor, S. K. Ahire Marg, Worli, Mumbai – 400030, Maharashtra, India.

PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes this invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
[001] The present invention relates to a process for extraction of phosphoric acid from rock phosphate. More particularly, the present invention relates to a process for extraction of phosphoric acid from rock phosphate showing increased efficiency of extraction by reducing the number of extraction cycles.
BACKGROUND OF THE INVENTION
[002] Phosphoric acid can be extracted from rock phosphate by two processes viz. wet process and thermal process. The thermal process normally produces a more concentrated and purer acid with huge energy consumption. The wet process involves reaction of rock phosphate with a mineral acid. This process is economical and practiced worldwide. However, the phosphoric acid produced by wet process contains a variety of impurities which are removed from the acid by several techniques such as precipitation, adsorption, ion exchange and solvent extraction.
[003] During solvent extraction process, the dissolved phosphoric acid is extracted in a series of well stirred mixer and settlers by addition of organic solvent through extraction steps followed by concentration. This results in phosphoric acid and calcium sulphate (gypsum) plus other insoluble impurities. A typical illustration of the process of extraction known in the prior art. The known process comprises of the following sequential steps: (1) Acidulation (2) Filtration (3) Decantation (4) Extraction (19 cycles) (5) Scrubbing (8 cycles) (6) Stripping (9 cycles) (7) Purification (4 cycles) (8) Concentration (9) Decolourization.
[004] In the prior art process, the extraction step utilizes about 19 contact cycles in counter current flow to meet the phosphorous oxide content in the spent brine i.e. less than 2.0 gpl and 8 contact cycles for scrubbing step with a view to purify loaded organic layer from co-extracted metal impurities and 9 contact cycles for stripping step to unload phosphoric acid from purified loaded organic layer. Therefore, the steps of extraction of phosphoric acid takes place in around 19 cycles which makes the process lengthy and inefficient. The extraction step is the heart of the phosphoric acid production process and have significant scope of improvement.
[005] Therefore, there is a need of a process for extraction of phosphoric acid from rock phosphate which solves the problem of the prior art.
SUMMARY OF THE INVENTION
[006] According to an embodiment of the present invention, there is provided a process for extraction of phosphoric acid from rock phosphate, comprising the steps of:
a) treating rock phosphate with hydrochloric acid to form a slurry;
b) filtering the slurry of step a) to separate a solid phase from an aqueous phase;
c) decanting the filtrate obtained in step b) to further remove the remaining solid phase;
d) adding organic solvent to the aqueous phase of step c) to extract phosphoric acid and remove metal impurities;
e) scrubbing the organic solvent with a weak phosphoric acid;
f) stripping phosphoric acid from the organic solvent using water; and
g) purifying the phosphoric acid by washing it with the organic solvent,
wherein the organic solvent is a mixture of tributyl phosphate (TBP) and isoamyl alcohol.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[007] The present invention relates to an efficient process for extraction of phosphoric acid from rock phosphate enabling the reduction in the number of extraction cycles from existing prior art process i.e. from 19 contact cycle to 8-9 cycles thereby offering an increase in the extraction efficiency from 70-75% to about 90-95%.
[008] The present process comprises of the following sequential steps: (1) Acidulation (2) Filtration (3) Decantation (4) Extraction (with reduced number of cycles) (5) Scrubbing (6) Stripping (7) Purification (8) Concentration (9) Decolourization.
[009] In a preferred embodiment, the present invention involves the usage of a novel organic solvent which is a mixture of tributyl phosphate (TBP) and isoamyl alcohol as an organic solvent that improves the extraction efficiency of phosphoric acid by utilizing less number of contacts cycles. Preferably, TBP and isoamyl alcohol are mixed in a ratio of 3:2 (v/v).
[010] According to an embodiment of the present invention, the process steps of the present process include - (Step 1) In a reactor, rock phosphate is treated with hydrochloric acid to form a slurry. (Step 2) This crude phosphoric acid slurry is then filtered to separate the solid phase from the aqueous phase. (Step 3) The remaining solid present in the filtrate is further separated out by decantation. This step removes around 30% of solid. (Step 4) Next step involves the extraction of phosphoric acid which involves around 5 cycles to extract the phosphoric acid from the aqueous phase. Preferably, a mixture of TBP and isoamyl alcohol is used as an organic solvent which increases the process efficiency by reducing the number of contact cycles. (Step 5) During scrubbing, the loaded organic layer is washed with weak phosphoric acid and later this impure weak acid is recycled back in the process. This step involves around 2 cycles. (Step 6) During stripping, phosphoric acid is unloaded from the organic solvent using water in around 2 cycles. The organic acid that is obtained in this step is recycled back in the process. (Step 7) The purification of dilute phosphoric acid is done in around 4 cycles in which it is washed with a solvent and metal impurities are removed. (Step 8) The concentration of phosphoric acid is increased by removing the mineral acid and water. The organic acid is recycled back in the process. (Step 9) The colour of phosphoric acid is removed thereby obtaining phosphoric acid with purity greater than 86%.
[011] The advantage of use of a new organic solvent includes a higher extraction of phosphoric acid from the aqueous phase.
EXAMPLES
[012] Advantages and benefits of the process of neutralisation of alkaline bauxite reside according to the embodiments of the present invention would become more apparent from the below experimental details to a person skilled in the art.
[013] Experimental Data 1:
3000 part of rock phosphate was digested with 5500 part of 33-36% hydrochloric acid in a stirred reactor. To control desired concentration of P2O5, water quantity was adjusted (0.5-0.9 part). The resultant colloidal mass was held to be settled on addition of flocculent solution. The decantation followed by filtration steps provided wet process acidic liquid. It was observed that the composition of source ore and the wet process acidic liquid contains:
Metal impurities level in ppm
Fe Mg Mn Zn Cu Cr Pb Ni
Rock Phosphate 5885 1216 257 276 252 137 4.0 21
Crude wet acidic liquid 1744 384 73 83 73 37 1 6
Thereafter, into a round bottom flask, 250 parts of wet process acidic liquid (~135 gpl of P2O5) and 550 parts of mixture of TBP and isoamyl alcohol with 50 parts of hydrochloric acid were added and stirred to achieve equilibrium. The mixture was transferred to separating funnel which allows mixture to separate in two layers. The separated lower aqueous raffinate layer was re-contacted with the fresh 550 parts of mixture of TBP and isoamyl alcohol along with mineral acid. The above process was repeated total five times in case of new formulated solvent whereas in the prior art process, it was carried out for ten contact cycles. Finally, the lower aqueous raffinate layer (spent brine) was analysed for the P2O5 content as well as for metal impurities co-extracted from acidic liquid to organic layer (Table 1).
Table 1

Solvent P2O5 content in spent brine (gpl) No. of Contact cycle % of metal impurities co-extracted by organic solvent
Fe Mn Mg Zn Cu Cr
A mixture of TBP and isoamyl alcohol 1.83 5 99.7 44 13.9 99.2 99.2 81.9
Solvent-1 2.20 10 100 51.9 14 99.5 77 80.2
Solvent-2 2.04 10 99.9 60.7 25.9 99.2 93.4 95.9
Solvent-3 2.27 10 99.9 64.5 35.9 99.4 96.8 97.6
[014] Experimental Data 2:
Similarly following above experimental procedure, acidic liquid (~135 gpl P2O5) was contacted one time with the mixture of TBP and isoamyl alcohol. The final raffinate aqueous layer (spent brine) was analysed for the P2O5 content and for trends of metal impurities co-extracted. It was observed that the mixture of TBP and isoamyl alcohol as an organic solvent in one contact cycle (Table 2) and even in three contact cycles (Table 3) extracted higher P2O5 content. The affinity of co-extraction for the iron, zinc and copper was higher in both organic solvents i.e. a mixture of TBP and amyl alcohol and Dow chemical`s Pentanol mixture and reached to a maximum level in one contact cycle while manganese and magnesium still had scope to build up its concentration in organic solvent. The iron metal co-extracted up to 94 to 99%, about 85-86% copper and 62-86% zinc was co-extracted in one contact cycle while about 6-9% magnesium and about 8-11% manganese was co-extracted in one contact cycle. The manganese and magnesium concentration was build up in the next cycles, therefore higher number of contact cycles would lead to build up Mg, Mn concentration in organic solvent.

Table 2
Solvent No. of Contact cycle P2O5 in spent brine (gpl) % of metal impurities co-extracted by organic solvent
Fe Mn Mg Zn Cu
A mixture of TBP and isoamyl alcohol 1 46.22 99 11 6 86 84.8
Solvent-1 (Dow chemical`s Pentanol mixture) 1 60.19 94 8 9 62 53

Table 3
Solvent No. of Contact cycle P2O5 in spent brine (gpl) % of metal impurities co-extracted by organic solvent
Fe Mn Mg Zn Cu
A mixture of TBP and isoamyl alcohol 3 4.4 99.7 26.7 17.4 99 98
Solvent-1 (Dow chemical`s Pentanol mixture) 3 13.56 97.6 14.6 14.4 93 95.3
Solvent-2 (Brazil Isoamyl alcohol) 3 9.85 99.8 43.3 20 87.3 33.9
Solvent-3 (Best value chemical Pentanol mixture) 3 9.44 99.8 20.9 17.6 96.2 91.31

[015] Further, the P2O5 loaded organic layer from extraction step of new formulated solvent and plant solvent-1 was scrubbed with soft weak phosphoric acid (P2O5 166 gpl) in a volume ratio aqueous to organic (1: 6) using two contact cycles. The percentage of translocation of co-extracted metal impurities from loaded organic layer to soft acid are given in Table 4. The purified loaded organic layer was treated with soft water in volume ratio 1: 4.5 (aqueous to organic) to unload phosphoric acid from purified organic layer to water in the said experiment total three contact cycle used and the status of metal impurities are given in Table 5.

Table 4
Solvent % of metal impurities back extracted by weak phosphoric acid
Fe Mn Mg Cu Zn
A mixture of TBP and isoamyl alcohol 73 23 8.7 73 26
Solvent-1 (Dow chemical`s Pentanol mixture) 95 13 8.9 95 30

Table 5
Solvent Metal impurities present in dilute phosphoric acid solution (ppm)
Fe Mn Mg Cu Zn
A mixture of TBP and isoamyl alcohol 64.9 0.1 2.0 0.3 10
Solvent-1 (Dow chemical`s Pentanol mixture) 2.1 0.1 0.7 0.2 8

[016] The foregoing description of specific embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obvious modifications and variations are possible in light of the above teachings.

,CLAIMS:We Claim:

1. A process for extraction of phosphoric acid from rock phosphate, comprising the steps of:
a) treating rock phosphate with hydrochloric acid to form a slurry;
b) filtering the slurry of step a) to separate a solid phase from an aqueous phase;
c) decanting the filtrate obtained in step b) to further remove the remaining solid phase;
d) adding organic solvent to the aqueous phase of step c) to extract phosphoric acid and remove metal impurities;
e) scrubbing the organic solvent with a weak phosphoric acid;
f) stripping phosphoric acid from the organic solvent using water; and
g) purifying the phosphoric acid by washing it with the organic solvent,
wherein the organic solvent is a mixture of tributyl phosphate (TBP) and isoamyl alcohol.
2. The process as claimed in claim 1, where tributyl phosphate (TBP) and isoamyl alcohol are mixed in the ratio of 3:2 (v/v).
3. The process as claimed in claim 1, wherein the weak phosphoric acid of step f) is recycled back in the process.
4. The process as claimed in claim 1, wherein the organic solvent obtained in step g) is recycled back in the process.
Dated this the 3rd day of January, 2020
For Grasim Industries Limited
By their Agent

(Vrinda Kaul)
Reg. No. IN/PA-2078